OPINTON

I. INTRODUCTION

This appeal presents our first opportunity to consider whether City & Borough of Ju-neam v. Thibodeau, 1 holding that a superior court decision remanding a case to an administrative agency is not a final judgment for purposes of appeal to thls court, 2 should apply to Alaska Workers' Compensation Appeals Commission decisions: We conclude that it should.

This appeal also presents our first qppor-tunity to consider, at least in part, the legis *906 lature's 2005 amendments to the Alaska Workers' Compensation Act's presumption analysis, We reverse the Commission's application of that analysis in this case and modify its earlier precedent.

II. OVERVIEW OF THE RELEVANT LEGAL ISSUES BEFORE US

A. City & Borough of Juneau v. Thibo-deau

Before the Commission's creation an Alaska Workers' Compensation Board decision could be appealed to the superior court, and a party dissatisfied with the superior court's final resolution of the case then could appeal to this court. 3 Construing the appellate rules, we decided in Thibodeau that "a decision of a superior court, acting as an intermediate appellate court, which reverses ... the decision of an administrative agency and remands for further proceedings, is a non-final order of the superior court." 4

In 2005 the legislature created the Commission, 5 a quasi-judicial agency in the executive branch 6 authorized to hear appeals from decisions of the Board, 7 and gave parties to a Commission decision the right to appeal a "final" decision to this court. 8 The legislature also authorized our review of other Commission orders "as provided by the Alaska Rules of Appellate Procedure." 9

In the case before us the Commission decided that its decision was final as to some issues but not as to others and gave the parties notice that they could appeal to this court those parts of the decision that were "final" but needed to petition for discretionary review if they wanted review of the "non-final" portions of the order. The injured worker appealed a "final" part of the decision. We ordered the parties to provide supplemental briefing on the question of the finality of the Commission's decision and the applicability of the Thibodeau rule to the Commission's decision.

B. Three-Step Presumption Analysis

In addition to creating the Commission the 2005 amendments to the Alaska Workers' Compensation Act changed the causation standard for compensable injuries. 10 At issue here is the effect of this change on the presumption analysis used to evaluate workers' compensation cases. -

1. Pre-2005 analysis

For work-related injuries before November 7, 2005, 11 application of the presumption of compensability consisted of three possible steps. 12 At the first step the employee was required to attach the presumption that the disability was work related by "establish[ing] a preliminary link between his disability and his employment." 13 To establish the link the employee was required to offer " 'some evi-denecee' that the claim arose out of the worker's employment." 14 If the employee attached the presumption, the burden shifted to the employer to offer substantial evidence that either (1) provided an alternative explanation excluding work-related factors as a substantial cause of the disability, or (2) "directly eliminated any reasonable possibility *907 that employment was a factor in causing the disability." 15 We called the two methods of rebutting the presumption "affirmative evidence" and "negative evidence." 16 An employer could rebut the presumption by presenting a qualified expert's testimony that the claimant's work was probably not a substantial cause of the disability. 17 The first two stages of the analysis required the Board to consider the evidence in isolation Wlthout weighing it. 18

If the employer presented enough evidence to rebut the presumption, the burden shifted back to the employee to prove the claim by a preponderance of the evidence. 19 Only at the third stage could the Board weigh the evidence. 20 The employee had to show by a preponderance of the evidence that work was a substantial factor in causing the disability: to prevail, the employee had to show that "(1) 'but for' the employment the disability would not have occurred, and (2) reasonable persons would regard the employment as & cause and attach responsibility to it." 21

2. The 2005 amendments

In 2005 the legislature repealed and reenacted AS 28.30.010, 22 modifying the standard for compensability of work-related injuries. The legislature also included in the reenacted statute: a presumption analysis formulation. Alaska Statute 23.30.010(a) now provides:

Exeept as provided in (b) of this section, [23] compensation or benefits are payable under this chapter for disability or death or the need for medical treatment of an employee if the disability or death of the employee or the employee's need for medical treatment arose out of and in the course of the employment. To establish a presumption under AS 23.30.120(a)(1) that the disability or death or the need for medical treatment arose out of and in the course of the employment, the employee must establish a causal link between the employment and the disability or death or the fieed for medical treatment. A presumption may be rebutted by a demonstra-of substantial evidence that the death or disability or thk need for medical treatment did not arise out of and in the course 'of the employment. When < determining whether or not the death or disability or need for medical treatment arose out of and in the course of the employment, the '[Bloard must evaluate the relative contribution of 'different causes of the disability 'or death or the need for medical treatment, Compenéation or benefits under this chapter are payable for the disability or death or the need for medical treatment if, *908 in relation to other causes, the employment is the substantial cause of the disability or death or need for medical treatment,

The legislature did not amend the definition of "arising out of and in the course of em-ploymenit" in AS 23.30.395 in 2005. 24

In Runstrom v. Alaska Native. Medical Center the Commission construed the, new statutory language as changing the presumption analysis only at the second and. third stages. 25 The Commission previously had interpreted the phrase "the substantial cause" in AS 28.30.010(a) as meaning that a disability is compensable if, in comparison to other causes, work is the most important factor in bringing about the disability. 26 In considering how the statutory change affected the second stage, the Commission in Runstrom decided that the negative-evidence test from our pri- or cages-"directly eliminat{ing] any reasonable possibility that employment was a factor in causing the disability" 27 —was now "incompatible with the statutory standard for causation" because under the amended statute "employment must be more than a factor in terms of causation." 28 It also determined in Runstrom that an employer can rebut the presumption by showing "that a cause other than employment played a. greater role in causing the disability ...." 29

The Commission decided here that the employer had met its burden through expert opinions not identifying an alternative cause but nonetheless concluding that work was probably not the substantlal cause of the employee's need. for medical treatment. The parties dispute whether the employer rebutted the presumption and in so doing raise the questlon of the 2005 amendments' impact on the second stage of the presumption analysis both generally-and as applied to this case.

III. FACTS AND PROCEEDINGS

Joseph Huit worked for Ashwater Burns, Inc. in 2010. Early in November he was working on a remodel project, and as part of the job -he removed a water-damaged vanity - from a bathroom. As he was carrying the vanity he .scratched his abdomen on a protruding drywall serew; he showed the serafch to some people at the job site, including his brother Steven, but did not file a report of i mJury

Late that mght Huit left Alaska to visit his daughter and grandchildren in Florida. Near the end of the visit his daughter noticed the scratch, which she thought was inflamed. Huit flew back to Alaska, stopping for a long layover in Seattle where he met with his wife, who had been caring for her father in Oregon. His wife also noticed the scratch and told him to watch it, According to Huit at some pomt later in November the scratch appeared to heal.

On Friday December 3 Huit felt ill at work, so he went to the emergency room. After testing Huit the doctor diagnosed a likely "viral syndrome" and told Huit to go home and rest but to return for a recheck if a fever still was present the following Monday. Huit stayed home for about five days, but his symptoms did not improve-they got worse. On December 9 he returned to the emergency room, where he was diagnosed with endocarditis; 30 he was hospitalized for several weeks while he received antibiotics to treat the infection, The emergency room physician wrote that Huit had "spontaneous endocarditis" 31 and commented that he "had *909 no history of IV drug abuse." Upon admission another physician noted that there was "[njo evidence of significant rash, erythema, breakdown, or bruising." An infectious-disease doctor was consulted as well; he observed that Huit's blood cultures were "growing Staphylococcus aureus," a type of bacteria, and reported "[mjetastatie lesions to the spleen, kidneys and brain" as well as "probable vegetation" on Huit's heart.

Studies of Huit's heart showed progressive damage. An echocardiogram in late December 2010 indicated "[mjloderate to severe aortic regurgitation," when two weeks earler there had been only mild regurgitation and the valve opened well. By January.2011 Huit had severe aortic regurgitation, and in February he underwent aortic valve replacement surgery.

Huit first thought about the possibility that the infection was work related while he was hospitalized; he explained that after the doctors told - him he had an infection, he remembered the seratch and notified his employer. Ashwater' Burns filed a report of injury on December 21 and later controverted benefits, relying on a cardiologist's opinion formed after reviewing Huit's medical records. One of Huit's treating physicians, Dr. Robert Bundtzen, an infections-disease specialist, noted on January 4, 2011 that an "abdominal abrasion" as described by Huit was "a possible portal of entry" for the staph bacteria. Huit filed a written workers' compensation claim for several benefits, including temporary total disability and medical costs, in early January 2011.

Ashwater Burns's employer's independent medical evaluation (EIME) consisted of ree-ords reviews by two physicians: a cardiologist and an infectious-disease specialist, Dr. Semler, the cardiologist, was skeptical that the seratch had happened or that it was the cause of the infection because "[a] screw does not cause Staph{ lylococeus infection. ... [I]t has never been reported in medical literature that Staphylococcus grows on serews." He concluded: "The more likely medical explanation for the cause of the bacterial endocar-ditis is unknown, not related to the speculated 'scerateh' injury if it occurred -at all." Dr. Leggett, the infectious-disease specialist, did not think the seratch "was a more probable than not substantial cause of [Huit's] S. au-reus aortic valve endocarditis," He acknowledged that with this type of infection "[the portal of entry may be rather insignificant, such as the alleged abrasion/serateh," but thought the infection was "just as likely to occur outside of work.as at work." He observed that the source.of the bacteria was Huit's "own skin," not the drywall screw. He also cited a study showing that "18% of S. aureus bacteremias" [32] had no identifiable source" and concluded Huit fell into that category. Dr, Leggett wrote that it was unlikely “an infected local wound" would, be visible three to four weeks later. He did not think the oufeome would have, been different if Huit had sought medical treatment when the scratch first: became inflamed.

Because of the difference in medical opinions about causation, the Board ordered a second ' independent medical evaluation (SIME), consisting of two examinations several months apart, one by Dr. William Breall, a cardiologist, and the other by Dr. Francis Riedo, an infections-disease specialist, Dr. Breall wrote that no evidence in the medical records "indicate[d] that the seratch on the abdomen caused a bacteremia"; to support this' statement, he noted that the serateh "did not produce pus," the scratch "was not infected," and "no culture [was] obtained from [the] seratch at the time that it was red in appearance." Dr. Breall concluded there was "no hard evidence to indicate that Mr. Huit had an industrial accident" that caused the infection, but he agreed with Dr. Bundtzen that the abdominal 'serateh was "a poss1ble portal of entry."

Dr. Riedo also thought it was possible but not probable that the "seratch was the substantial cause of Mr. Huit's endocarditis." He thought it was "medically reasonable that a scratch as described" could cause endocardi-tis, but he also thought a seratch of that nature would still have been visible three to four weeks later: Dr. Riedo did not think Huit's work-related "injury or condition ag-gavate[d], combine[d] with or accelerate[d] *910 any condition whose treatment or disability [was] not otherwise" work related, He concluded, "I do not believe that the purported seratch or infected seratch was the source of Mr. Huit's infection on a more probable than not basis."

Huit moved to Idaho and began treating with Dr. Dennis Stevens at the Boise Veterans Administration (VA) beginning in 2013. 33 «Based on Huit's reports the VA medical staff thought the work-related seratch was 'the likely cause of Huit's endocarditis.

The Board held a hearing solely about the compensability of Huit's illness. Several lay witnesses testified, but no doctors did. The focus of the testimony was the witnesses' observations of Huit's abdominal seratch, At the conclusion of the hearing Huit argued that he had attached the presumption and his employer had not rebutted it because the doctors the employer relied on could not eliminate work as a causal factor in his need for medical treatment and subsequent disability. The employer argued it had rebutted the presumption through expert reports that concluded work was not the substantial cause of Huit's endocarditis. It also contended Huit had not reported the injury in a timely manner. ==

To resolve the compensability question, the Board first considered whether Huit had in fact scratched himself at work. The Board decided this was not a complex medical question, and it applied the three-step presumption analysis to this factual question. 34 It found that Huit had attached the presumption through his testimony and the testimony of his brother Steven. The Board determined that Ashwater Burns had rebutted the presumption with a chart note from the hospital "that [there is no good portal of entry'" and with a medical opinion that the seratch would still have been visible on December 9, the date Huit was admitted to the hospital. The Board then weighed the evidence, giving the medical testimony less weight because doctors had given completely different opinions about whether a seratch like the one Huit described would still be visible when he went to the emergency room. The Board gave the lay testimony more weight; it found that Huit had suffered a seratch in the course and seope of his employment with Ashwater Burns.

The Board then turned to the endocarditis. The Board found Huit had attached the presumption the endocarditis was work related through Dr. Stevens's opinion that the scratch was "the only potential portal of entry for the infection." The Board interpreted prior Commission decisions as requiring Ash-water Burns to "present substantial evidence that a cause other than the seratch played a greater role in causing the infection" to rebut the presumption. The Board examined different doctors' testimony. It first considered Dr. Semler's testimony; according to the Board, he "clearly doubted the seratch occurred" but identified the "more likely medical explanation for the cause of the bacterial endocardi-tis [was] unknown." The Board did not consider an unknown cause to be "substantial evidence that a cause other than employment played a greater role in causing the need for medical treatment." Both Dr. Breall, the SIME cardiologist, and Dr, Leggett, the employer's infectious-disease specialist, acknowledged that a seratch like Huit de-'seribed' could 'be a portal of entry for the bacteria, but neither doctor thought the seratch was more probably than not the substantial cause of the endocarditis. Both doe-tors said it was just as likely the bacteria had entered Huit's, bloodstream through another scratch or skin lesion, but neither doctor pointed to any evidence that Huit had any such other problem. Again, the Board did not consider these doctors' opinions substantial 'evidence that a cause other than Huit's work "played a greater role in causing [his] need for medical treatment." Finally, the Board summarized Dr. Riedo's testimony. Dr. Riedo agreed with the other doctors that the *911 scratch was a possible entry portal for the bacteria, but he thought the infection "was "impossible to attribute to a single event." The Board did not consider this opinion substantial evidence that could rebut the presumption because "if it is impossible to attribute the infection to a single event, it cannot be attributed to a cause other than work."

The Board wrote that to rebut the presumption here, the employer needed to produce more evidence than opinions that more likely than not, the seratch was not the cause of the illness, because the doctors were unable to identify any other cause of the illness. The opinions all considered the cause to be unknown, even though they agreed the bacteria somehow had to have entered Huit's bloodstream. Because the Roard did not consider any of the doctors' opinions sufficient to rebut the presumption, it decided that Ash-water Burns had not met its burden at the second stage of the presumption analysis, making Huit's illness compensable.

The Board then engaged in an alternative analysis, in which it assumed the employer had rebutted the presumption. The Board gave the most weight to the opinions of Drs, Bundtzen and Stevens, Huit's treating physicians. After again noting that several doctors agreed the seratch was a possible entry point for the bacteria but doubted the existence of the scratch, the Board gave less weight to the opinions of Drs. Breall, Riedo, Leggett, and Semler because they "lacked important, credible evidence about the existence of the scratch." Based on the evidence and the weight assigned to it, the Board concluded that Huit had proved by a preponderance 'of the evidence that his work injury was the substantial cause of the endocarditis.

Ashwater Burns appealed to the Commission, which affirmed in part and reversed in part. The Commission agreed with the Board that Huit's claim was timely filed and that he had attached the presumption of compensa-bility. The Commission disagreed with the Board's legal analysis related to rebutting the presumption, labeling the Board's interpretation of the Commission's decision in Runstrom v Alaska Native Medical Center 35 "a narrow reading" that was nonetheless "understandable, given particular wording in that decision." The Commission faulted the Board for requiring the employer to produce substantial evidence that a cause other than employment was the substantial cause of Huit's endocarditis, The Commission decided that the presumption could be rebutted "through the presentation of substantial evidence that work was not the substantial cause of a disability." The Commission thought the evidence "ruled out what was identified as the one-and-only potential work-related cause of Huit's disability, namely the seratch." It quoted Dr, Riedo's.opinion that he did "not believe on a more probable than not basis that the November 5, 2010 seratch was the substantial cause" of the endocardi-tis. Dr. Riedo did not think it was probable because of "the lack of any skin lesion noted just three to four weeks after the serateh." The Commission also cited Dr. Breall's opinion that while it was possible that the scratch might have been the portal of entry, it was not probable. The Commission thought that these two opinions ruled out the scratch as the substantial cause of Huit's endocarditis, so it reversed the Board's determination that Ashwater Burns had not rebutted the presumption of compensability.

The Commission also found fault with the Board's alternative analysis. Specifically the Commission was "unable to identify an opinion from any of the medical experts ... that the scratch was, to a reasonable degree of medical probability, the substantial cause of [Huit's] infection and endocarditis." The Commission thought there was "an insufficient connection between the evidence and the [Bloard's conclusion." It reversed the Board's alternative decision that Huit had proved his claim by a preponderance of the evidence, and it remanded the case to the Board for "sufficient findings, based on the record from the hearing ... that would enable [the Commission] to effectlvely review" the Board's decision.

*912 The. Commission informed the parties that its decision was final as to its "affirmation of the [Bloard's decision in part, reversal of the [Bloard's decision in part, and vacating the [BJloard's decision in part." It also said the decision was "non-final" with respect to the "remand of the matter in part to the [Bloard." The Commission then notified the parties that the "final decision portion" of the decision became effective "when distributed" unless one of them filed a request for reconsideration or an appeal, The Commission advised. the . parties they, could petition this court for review of the non-final part of the decision.

Huit appealed the part of the Commission's decision about rebutting the presumption of compensability, The parties jointly asked the Board to decide whether it had jurisdiction "to consider the portion of the case remanded to the [BlJoard" in light of the appeal to this court. The Board decided that it had no jurisdiction over any part of the case while an appeal was before this court.

IV. STANDARDS OF REVIEW

' In an appeal from the Alaska Workers' Compensation Appeals - Commission, we review the Commission's decision rather than the Board's. 36 "We apply our independent judgment to questions of law that do not involve agency expertise." 37 Interpretation of a statute is a question of law to which we apply our independent judgment, interpreting a statute "according to reason, practicality, and common sense, considering the meaning of the statute's language, its legislative history, and its purpose." 38 We do not mechanically apply the plain meaning rule but use a sliding scale approach to statutory interpretation, in which "[the plainer the statutory language is, the more convincing the evidence of contrary legislative purpose or intent must be." 39 We review de novo the Commission's legal conclusion that substantial evidence supports. the Board's factual findings by "independently review[ing] the record and the Board's factual findings." 40

V. DISCUSSION

A. The Commission Decision Was Not A Final Decision For Purposes Of An Appeal As A Matter Of Right.

The Commission called its decision as to three issues "final" and gave the parties notice that the "final" decision parts would take effect unless they appealed to this court. Huit appealed the Commission's decision that Ashwater Burns had rebutted the presumption of compensability. Under the rule in City & Borough of Juneau v. Thibodeau-a superior. court decision remanding a case to an administrative ageney is not a final judgment for purposes of appeal to this court 41 —the Commission's decision would not be a final decision because the Commission remanded the case to the Board for further factual findings. 42 We ordered the parties to provide *913 supplemental briefing on Thibodeau's applicability to Commission decisions; we' also invited the State to participate as amicus curiae, which the State did.

Ashwater Burns and the 'State correctly observe that Thibodeau does not directly control when a Commission decision'is final for purposes of appeal because Thibodeau concerned appeals from superior court orders 43 and the Commission is an administrative agency rather than a court. 44 Huit and Ashwater Burns both contend that policy and our prior cases favor extending the rule in Thibodeau to Commission decisions; they ask us to treat this case as a granted petition, The State sets out several alternatives for determining when a Commission decision is final for purposes of appeal, but it takes no position as to the best one because of the many roles it has in the workers compensation process. ©

~ Our analysis begins with the statute's language; AS 28.30.129(a) provides in part: "Consistent with AS 22.05.010(b), [45] final decisions of the commission may be appealed to the supreme court, and other orders may be reviewed by the supreme court as provided by the Alaska Rules of Appellate Procedure." The legislature did not in this statutory subsection delineate what a "final decision of the commission" is, although it clearly envisioned a system in which some Commission decisions are final and others are not, Alaska Statute 28.80.128(e) requires the Commission to issue a written decision, with specific features, within 90 days of the completion of briefing or oral argument in an appeal from the Board; that written decision is called "the final commission decision."

One possible construction of the statute is to consider any decision having the characteristics set out in AS 23.30.128(e) a final decision for purposes of appeal This construction would be in accordance with the presumption that the same word used twice in the same act has the same meaning, 46 and it would make the entire decision in this case appealable as a matter of right. But this construction is inconsistent with our prece-. dent about the finality of administrative decisions for purposes of appeal to the superior court 47 and appears to be at odds with the Commission's normal practice when remanding the entire case to the Board. 48 Construing the two phrases as having the same meaning would also be contrary to the rule of statutory construction that terms used in statutes that have developed a specialized meaning through case law are presumed to have that specialized meaning 49 to and the corollary that the legislature is aware of existing case law when it enacts of modifies the law. 50

We have held that the test for finality for purposes of judicial review of administrative *914 decisions "is essentially a practical one" 51 that considers "whether the ageney has completed its decisionmaking process[] and whether the result of that process is one that will directly affect the parties." 52 We have instructed that in deciding whether an agency decision is final for judicial review, the superior court should "focus primarily on the operational or 'decretal' language" in the decision. 53 The inquiry should also consider whether the litigants still have an opportunity "to submit evidence or alter the decision through administrative means." 54 Because the Commission is a quasi-judicial ageney, we assume the legislature was aware of cases defining an ageney decision's finality for purposes of judicial review when it enacted AS 23.30.129 55 and intended a "final" decision in that section to have the same meaning.

Two agencies are involved in the administrative process of a workers' compensation decision rather than one, but they are integrally related, with Commission decisions serving as legal precedent for both agencies. 56 And while the Commission may complete its decision-making process in cases like this one, when it remands the case to the Board without retaining jurisdiction, 57 the entire administrative decision-making process is not complete if the remand permits further Board action that could change the case's outcome. On remand the parties here would have had the opportunity to alter the decision through administrative means, either through argument or presentation of additional evidence if the Board considered that necessary. As the Board recognized, the remand considered essentially the same underlying issue that this appeal does. The de-cretal language here contemplated further administrative proceedings by remanding so the Board could make further findings. Thus under our precedent the case was not a final administrative decision for purposes of judicial review.

Construing "final decision of the commission" in AS 28.30.129(a) as equivalent to a "final commission decision" in AS 23.30.128(e) would permit appeals as a matter of right in cases that do not meet our precedents' finality test and have not in the past been labeled "final" by the Commission. A case remanded on all issues to the Board is not "final" for purposes of judicial review because on remand the parties might present evidence and make arguments that could change the decision, The Commission has not treated such cases as "final," 58 yet they meet the eriteria of AS 28.30.128(e) as long as they are in writing and contain the features set out in that subsection.

Conversely, construing these phrases in the same manner could exclude decisions that meet our precedents' finality test, Alaska Statute 23.30.128(e) ties finality to the Commission's resolution of the merits appeal; a "final commission decision" is one the Commission issues after "briefing on the appeal is completed or oral argument is held." But the Commission awards attorney's fees under AS *915 28.30.008(d) as part of the merits appeal, not as 'a separate case, 59 so a decision about attorney's fees might never be "final" as the term is used in subsection. 128(e). 60 The Commission has not been consistent in giving notice of appeal rights in its decisions about fees. In one case it labeled a fee order "final" and gave the parties notice of their right to appeal; 61 in another case, it called its decision a "memorandum" decision and gave no notice to the parties about finality or. any appeal rights, even through discretionary review. 62 We conclude that the terms used in AS 28.80.128(e) and AS 23.30.129(a) are not equivalent. Finality for the Commission's purposes can be distinct from finality for purposes of judicial review; as the judicial body reviewing the Commission's administrative decisions, we will decide whether a Commission decision is final for purposes of judicial review by applying prior case law.

Our construction of the statute is consistent with the limited legislative history concerning the Commission. As we have observed previously, the legislature's goals in creating the Commission were to make the workers' compensation administrative process more expeditious and consistent. 63 Applying the rule in Thibodeau to Commission decisions furthers these goals by permitting discretionary review by this court when a case presents an important legal question but allowing the administrative process to come to completion prior to an appeal as a matter of right. We also have noted that the legisla-

ture wanted litigants appealing to the Commission to retain "the same procedural rights of review that they had in the superior court." 64 The rule in Thibodeou provides parties in the workers' compensation system the same process that existed prior to 2005; it also provides, as the State puts it, "an attractive symmetry," making the process more consistent with that of other agency appeals and thus easier for unrepresented parties to follow, Applying: Thibodeau also limits piecemeal appeals 65 and permits complete development of the agency record before judicial review, 66 Thibodeau allows a party 'to preserve legal issues decided throughout, the case for review by this court after completion of, the administrative process. 67

We are mindful that Commission decisions have the foree of legal precedent for both the Board and the Commission unless reversed by this court; 68 the State pointed to this feature of the statutory scheme as one policy consideration weighing against application of Thibodeau to Commission decisions. Parties to an appeal like this one, involving an important question of law and its application to the case, can petition for review of a Commission decision if they are concerned about the Commission's legal analysis. 69 And the legislature both provided a mechanism for the director 'of the Division of Workers' Compensation to appeal in Commission proceedings and permitted the Division's director to ap *916 peal a compensation order.to the Commission when a party in interest is not represented and the order "concerns an unsettled question of law." 70 These procedures should adequately protect against the risk that a legally erroneous Commission: decision would taint numerous cases.

In light of the Commiskion's notice to the parties here about their appeal rights, Huit understandably appealed the Commission's decision that 'Ashwater Burns rebutted the presumption. Under the rule we adopt today, he 'did not have an appeal as of right. But because this case involves an important question of law and immediate review will materially advarice the termination of the litigation, 71 we follow our precedent to treat the appeal as a petition for review 72 and grant it.

B. The Commission Erred In Deciding , That Ashwater Burns Rebutted The Presumption Of Compensability.

The Commission reversed the Board's decision at the second stage of the presumption analysis and decided that Ashwater Burns had rebutted the presumption of com-pensability. The Commission thought the Board's interpretation of the Commission's prior decisions was unduly "narrow" and wrote that "the presumption can be rebutted through the presentation of substantial evidence that work was not the substantial cause of a disability." The Commission explained that the question the Board needed to consider was "whether the medical evi-denee in this case ruled out employment as the substantial cause of Huit's infection and endocarditis." Focusing on statements in the SIME physicians' reports, the Commission decided Ashwater Burns had provided substantial evidence to rebut the presumption. The Commission relied on Dr. Breall's opinion that he could not say "there was 'a reasonable medical degree of probability' that the scratch" caused Huit's "Infection, even though Dr. Breall acknowledged it was possible the seratch could have been where the

staph bacteria entered Huit's bloodstream. It also quoted Dr. Riedo's opinion that "Iwlhile it is medically reasonable that a seratch as described by Mr. Huit can cause this 'illness, it is possible but again not' probable" because no skin lesions had been documented 'three to four weeks after the seratch. - -

Huit argues that the presumption analysis as applied to his injury should be no different from the presumption analysis before the 2005 statutory changes, set out in Section II.B.1, because the legislative changes were meant to apply to claims where a work injury aggravated a preexisting condition or injury and he had none. He maintains that because there is no cause with which to compare the work-related scratch, Ashwater Burns did not rebut the presumptlon because it could not rule work out ss his infection's cause; in fact, the doctors all agreed the scratch was a possible entry point for the bacteria that caused the infection,

Ashwater Burns contends the Commission correctly decided that medical opinions in the record rebutted the presumption and that to decide otherwise would create an irrebutta-ble presumption. It asserts that because the statute now unambiguously provides that compensation is payable only when, in relation to other causes, employment is "the substantial cause" 73 of a disability or need for medical treatment, at the second stage it only needed to provide an expert opinion that the seratch was not the substantial cause. of the infection.

. As stated previously, we have yet to construe the 2005 amendments to the Alaska Workers' Compensation Act as they relate to the presumption analysis, The only part of the presumption analysis at issue in this appeal is the second stage: Ashwater Burns does not contest that Huit attached the presumption, and Huit did not ask us to review the Commission's decision about the third stage,

*917 Under the pre-2005 analysis the employer could rebut the presumption of work-relatedness by presenting substantial evidence that either (1) provided an alternative explanation that would exclude work-related factors as a substantial cause of the disability, or (2) directly eliminated any reasonable possibility that employment was a factor in causing the disability. 74 An employer could rebut the presumption by presenting a qualified expert's opinion testimony that the claimant's work was probably not a substantial cause of the disability. 75 That opinion had to be supported by substantial evidence. 76

With respect to the second stage, AS 28.80.010(a) now provides that the "presumption may be rebutted by a demonstration of substantial evidence that the death or disability or the need for medical treatment did not arise out of and in the course of the employment." 77 It then instructs: "When determining whether or not the death or disability or need for medical treatment arose out of and in the course of the employment, the [Bloard must evaluate the relative contribution of different causes of the disability or death or the need for medical treatment." |

We begin our analysis by examining the statutory language. The statutory language does not require the Board to determine or even consider "the substantial cause" at the second stage of the presumption analysis, but it does require the Board to "evaluate the relative contribution of different causes" in deciding whether the disability arose out of and in the course of employment. Here no other cause was identified as contributing to Huit's infection, so the Board did not need to evaluate the relative contribution of different causes to the infection, The Board nonetheless was required to consider whether Ash-water Burns had provided "a demonstration of substantial evidence that the ... disability or the need for medical treatment did not arise out of and in the course of the employment." 78

Ashwater Burns maintains that it dld so through medical opinions that, on a more probable than not basis, the scratch was not the substantial cause of the disability. But the statute does not instruct the Board to make the determination of "the substantial cause" at the rebuttal stage. And at no point does the statute explicitly equate "aris[ing] out of.and in the course of employment" with "the substantial cause." 79 While both phrases are prerequisites for receiving workers' compensation, 80 they could be two independent conditions rather than one, equivalent condition We thus disagree with Ashwater Burns's contention that the statutory language in this regard is not ambiguous. To resolve the ambiguity, we examine the legislative hlstory to consider the degTee to which the legislature intended to modify the presumption analysis developed under the prior causation standard.

As Ashwater Burns points out, "the substantial cause" as a standard for awarding compensation originated with the legislature's desire to limit aggravation claims. One principal reason the governor cited for proposing the 2005 Alaska Workers' Compensation Act amendments was "increasing costs of maintaining the current system." 81 The legislature considered limiting the compensa-

*918 bility of aggravation claims as a means of reducing insurance costs. 82 The Senate Judiciary Committee proposed added language that would have narrowed the definition of "injury" to exclude from coverage an "aggravation, acceleration or combination with a preexisting condition unless the employment [was] the major contributing cause of disability or need for medical treatment." 83 According to Paul Lisankie, then the Director of the Division of Workers' Compensation, "the major contributing cause" would be the predominant cause of the disability, or at least a 51% cause. 84 He indicated that "the major contributing cause" standard was meant to establish a "higher standard" for compensability than the current law and that it was derived from Oregon Law. 85

The House Labor and Commerce Committee removed this definition of "injury," 86 but the change in defining "injury" was reinserted in the bill's first conference committee version. 87 The House did not adopt the conference committee version of the bill. 88 In the Free Conference Committee, Senator Gene Therriault proposed an amendment specifically providing that compensation was not payable for aggravation claims unless employment was "the major contributing cause" of the disability; that amendment failed. 89

Senator Therriault then proposed an amendment that established the language of AS 23.30.010. 90 Senator Therriault indicated the amendment's language was developed with Assistant Attorney General Kristin Knudsen, who spoke to the committee about the bill. 91 Knudsen testified that the language about the operation of the presumption was derived from prior cases and was not intended to change the way the presumption analysis operated. 92 She identified both tests in prior case law for overcoming the presumption, 93 and she indicated, in response to Senator Hollis French, that the language up through the last line-in other words, the language about application of the presumption analysis-was "not intended in any way to restrict or change the current standard for work relationship." 94 Knudsen and Senator Therriault both told committee members that the language about attaching and rebutting the presumption was derived from our case law," 95 and comments of some committee members indicate they understood the amendment as codifying the standards for attaching and rebutting the presumption. 96 *919 The legislative history thus suggests application of the presumption analysis was to remain intact; there is no indication the legislature intended to change the manner in which an employer rebutted the presumption.

Based on the legislative history we conclude that the Commission's interpretation of the statute as abrogating the negative-evidence test from prior case law was erroneous, 97 and we overrule that part of the Commission's Runstrom decision. 98 We agree with the Commission's earlier observation that an opinion establishing that a cause is not a substantial factor of the disability rebuts the presumption using either "a substantial factor" or "the substantial cause" as a standard because something cannot be "the substantial cause" of a disability if it is not a cause at all. 99 We observe that elimination of the negative-evidence test arguably made it harder for an employer to rebut the presumption in those cases of medical uncertainty in which the underlying condition is poorly understood. 100

We next consider whether Ashwater Burns's evidence met either the affirmative-evidence test or the negative-evidence test. Because there was no competing cause in this case, we do not consider how the presumption analysis should be applied when another possible cause, such as a prior injury, contributed to the disability, We agree with Huit that when there is no competing cause, the standard for rebutting the presumption is essentially unchanged from prior cases: the requirement in subsection .010(a) that the Board "evaluate the relative contribution of different causes" when assessing work-relatedness presupposes the identification of more than one cause.

Under the amended statute, rebutting the presumption required Ashwater Burns to show that Huit's infection did not arise out of his employment. On the facts of this case, to do this, Ashwater Burns needed to show that the work-related seratch could not have caused the infection (the negative-evidence test) or another source of the bacteria caused the infection (the affirmative-evidence test). It argues that because experts gave the opinion that work was probably not the substantial cause of the disability it provided this evidence. But, as we said in Sofeway, Inc. v. Mackey, "merely reciting the proper words as an opinion is not necessarily enough to rebut the presumption of compensability, because the employer must provide substantia-levidence that the disability was not work-related," 101

Considering first the negative-evidence test, we conclude that the doctors' opinions do not meet this test; they do not show that the work-related seratch could not have been the entry point for the bacteria that caused the infection. In fact the experts indicated that bacteria can enter the bloodstream through minor scratches like one Huit described, and Ashwater Burns conceded at oral argument before us that the scratch was a possible entry point for the bacteria. 102 Moreover the Commission cited the opinions of Drs. Breall and Riedo to support its determination that Huit attached the presumption.

*920 The doctors' opinions here are distinct from the negative evidence offered to rebut the presumption in Norcon, Inc. v. Alaska Workers' Compensation Board 103 and Safeway, Inc. v. Mackey, 104 two cases on which Ashwater Burns relies. In both Norcon and Mackey, the employee established a causal connection, but it was eliminated by expert-opinion testimony that no relationship existed between the alleged cause and the disability. In Norcon, where long hours and their resulting stress allegedly caused a worker's ventricular fibrillation and resulting death, a doctor rebutted the alleged causal connection by testifying that working long hours "is not recognized to be a risk factor for sudden cardiac death" and that there was no reasonable possibility "that the two are related." 105 And in Mackey, where the worker alleged that her fibromyalgia developed from earlier work-related injuries and repetitive stress, we recognized that the employer's doctor "testified that trauma and the development of fibromyalgia have not been reliably related" and "rebutted the theories that [the claimant's doctor] presented to link [the claimant's] work with her fibromyalgia," 106 In contrast the experts here indicated that a scratch like the one Huit described could be an entry point, Because they did not rebut the causation theory Huit's doctors presented, their opinions did not meet the negative, evidence test for rebutting the presumption.

For the doctors' opinions in this case to meet the affirmative-evidence standard, they needed to provide substantial evidence ruling out the work-related seratch as the source of the staph bacteria that caused Huit's endo-carditis by identifying another explanation for the bacteria's presence in Huit's bloodstream. 107 The Board carefully considered each doctor's opinion; we agree with the Board that none of the doctors provided substantial evidence of another cause.

"The mere possibility of another injury is not 'substantial evidence sufficient to overcome the presumption." 108 As the Board noted, Dr. Semler, the EIME cardiologist who based his opinion "on the lack of evidence to support the alleged serateh," said the "more likely medical explanation for the cause of the bacterial endocarditis is unknown." We agree with the Board that an "unknown" cause is not substantial evidence to rebut the presumption. The other EIME physician, Dr. Leggett, gave the opinion that "an unidentified source" was the substantial cause of Huit's endocarditis, even though he acknowledged that "[tlhe portal of entry may be rather insignificant, such as the alleged abrasion/seratch." Dr. Leggett listed other conditions that might provide entry points for the bacteria, but as the Board observed, "he did not identify [Huit] as having any of those conditions." His opinion, like that of Dr. Semler, was not substantial ev1dence re-buttlng the presumptlon

The SIME-physicians’ opinions were similar, While both agreed that the seratch could be an entry point for the bacteria, they.refused to identify it as the substantial cause of the infection, evidently because they lacked informatibn about or doubted the existence of the scratch. And like Drs. Semler and Leg-gett, Drs. Breall and Riedo were unable to identify another entry point. Dr. Breall observed that "staph aureus is ubiquitous" and can enter the bloodstream "in a susceptible individual from just about any place." He did not explain what would make an individual "susceptible," and, absent anything identify"ing Huit as particularly susceptible, this explanation does not rule out the work-related scratch as the cause of the staph infection. Dr. Riedo also accepted that the seratch Huit described could lead to endocarditis, but he thought- the: endocarditis was "impossible to attribute to a single event." He did not, however, point to other events that contributed to it.

*921 The evidence the Commission cited was not substantial, The opinions offered alternative theories of causation, but "the mere possibility" of a non-work-related alternative is not sufficient to rebut the presumption. 109 There was no direct evidence to support the existence of another seratch or any other possible portal for the bacteria, and, as the Board observed, the EIME and SIME doe-tors did not have access to "the credible lay testimony" about the existence of the seratch. 110 We thus conclude the Commission erred in deciding that Ashwater Burns rebutted the presumption of compensability.

C. There Is No Irrebuttable Presumption Here.

Ashwater Burns maintains that Huit seeks to create an irrebuttable presumption and that we have previously decided in cases of medical uncertainty that a doctor's opinion that the disability was not work related is adequate to rebut the presumption. It argues that several doctors' opinions met that standard, making the Commission's decision correct. Huit denies seeking to create an irre-buttable presumption, pointing out facts that might have rebutted the presumption,, such as evidence that he actually had "a similar injury away from work."

We agree with Huit that the difficulty Ashwater Burns faces is not a legal hurdle, but a factual one. The uncertainty in Huit's case revolved around where the bacteria entered his bloodstream. The experts all agreed that staph caused his bacterial infection, which in turn led to his need for medical treatment. They also indicated even minor seratches can serve as entry points for bacteria. The uncertain causation in the case was related to the existence of the scratch, as is evident from the experts' reports; the doe-tors expressed uncertainty about the existence of the scratch, not about whether bacteria could enter the bloodstream through a scratch. The Board separately analyzed the fact question related to the seratch and found that Huit had in fact been seratched at work as he alleged. Ashwater Burns did not question the Board's proceeding in this manner and did not appeal the Board's resolution of this factual dispute to the Commission.

The additional cases Ashwater Burns relies on to support its argument about irre-buttable presumptions are distinguishable. 111 In Cowen v. Wal-Mart the medical uncertainty involved whether physical . activity could cause a breast implant to deflate. 112 The employer's doctor testified that no one knew whether deflation was "related to physical. activity," but nonetheless gave the opinion, based on his experience, that work-related activities were probably not a cause in the deflation of the employee's implant. 113 We agreed with the Board that from this testimony reasonable minds could conclude that work-related physical activities were not a substantial factor in causing the employees disability. 114 Here, in contrast, the doctors agreed that Huit's scratch could provide an entry point for the staph and did not suggest an alternative entry point. Their opinions that the scratch was, on a more probable than not basis, not the substantial cause of the infection were related to doubts about the serateh's existence.

Likewise, in Bradbury v. Chugach Electric Ass'n an expert testified there was no way to quantify how much external pressure was needed to rupture an employee's preexisting cyst. 115 But the expert testified that sufficient trauma to do so usually came from blunt trauma, that heavy lifting could not cause a *922 rupture, and that the employee's work duties would not cause sufficient trauma to rupture the cyst. 116 We upheld the Board's determination that a reasonable mind could conclude from this evidence that work was not a cause of the cyst's rupture because there was no evidence of trauma and the expert directly refuted the employee's theory that her work duties put enough pressure on the cyst to rupture it. 117

An irrebuttable presumption is one "that cannot be overcome by any additional evidence or argument because it is accepted as irrefutable proof that establishes a fact beyond dispute." 118 Huit's case does not involve an irrebuttable presumption, even though Ashwater Burns's task in rebutting the presumption became more difficult after the Board made its finding about the scrateh's existence. In the end Huit's case is no different from cases we have previously considered, such as Firemen's Fund American Insurance Cos. v. Gomes 119 or Excursion Inlet Packing Co. v. Ugale. 120 In those cases the cireumstances of the employees' deaths were unknown, making the employer's task in rebutting the presumption a difficult one. 121 But the uncertainty in this case, as in Ugale or Gomes, is resolved through the use of the presumption of compensability. With different facts Ashwater Burns may have faced a less difficult task, but difficulty in finding and presenting additional evidence is not equivalent to creating an irrebuttable presumption.

VI. CONCLUSION

We REVERSE the Commission's decision that Ashwater Burns rebutted the presumption of compensability and REMAND to the Commission with instructions to reinstate the Board's award.