California Labor Code § 4663

Determinative passage · p.10Therefore, a medical opinion concerning apportionment must explain the facts relied on and the reasoning resulting in its conclusions. This is important because it allows the lay trier of fact to evaluate the substantial nature of medical evidence.

Applied to reject Dr. Robinson's apportionment opinion for lack of adequate explanation and reliance on speculation.

Determinative passage · p.6II. In the en banc decision in Nunes v. State of California, Dept. of Motor Vehicles (June 22, 2023) 2023 Cal. Wrk. Comp. LEXIS 30 [88 Cal.Comp.Cases 741] ("Nunes I"), the Appeals Board held that section 4663 requires a reporting physician to make medical determinations in a case, including determinations on the issue of apportionment. The Board further held that vocational evidence may be used to address issues relevant to the determination of permanent disability, and that vocational evidence must address apportionment, but that a vocational evaluator may not opine on issues that require expert medical evidence. The Board affirmed these holdings in Nunes v. State of California, Dept. of Motor Vehicles (August 29, 2023) 23 Cal. Wrk. Comp. LEXIS 46 [88 Cal.Comp.Cases 894] ("Nunes II").

Applied to reject vocational expert's apportionment opinion and require medical evidence.

Determinative passage · p.6In addition, section 4664 provides that "[i]f the applicant has received a prior award of permanent disability, it shall be conclusively presumed that the prior permanent disability exists at the time of any subsequent industrial injury." (Lab. Code, § 4664 (b).) "Section 4664(b) creates a conclusive presumption of the continued existence of a prior permanent disability when the claimant received an award of permanent disability benefits based on that disability," and that defendant "has the burden of proving overlap between the current disability and the previous disability in order to establish its right to apportionment...." (Kopping v. Workers' Comp. Appeals Bd. (2006) 142 Cal.App.4th 1099, 1115 [71 Cal.Comp.Cases 1229].)

The WCAB found the medical opinions did not meet the substantial evidence standard for apportionment because they lacked adequate explanation of causation and overlap.

Determinative passage · p.16A physician must determine apportionment based on causation, because an employer is liable solely for the portion of permanent disability directly attributable to an injury arising out of and occurring in the course of employment. (Lab.Code, § 4663(a) and 4664(a).) "The plain reading of `causation' in this context is causation of the permanent disability." (Escobedo v. Marshalls (2005) 70 Cal.Comp.Cases 604, 611 (Appeals Board en banc) (Escobedo).) Apportionment now includes pathology, asymptomatic prior conditions, and retroactive prophylactic work preclusions, provided there is substantial evidence establishing that these other factors have caused permanent disability. Pursuant to Escobedo, a physician's opinion must constitute reasonable medical probability, must not be speculative, rely on pertinent facts and/or an adequate examination and history, and must set forth the reasoning in support of the conclusions. (Id. at p.621.) That is, a physician must explain the "howand why" of their apportionment opinion and consider all potential causes of disability, whether from a current, prior or subsequent industrial or nonindustrial injury or condition. (Ibid.; Benson v. Permanente Med. Group (2007) 72Cal.Comp.Cases 1620, 1622 (Appeals Board en banc).)

Applied to reject Dr. Sussman's apportionment opinion as speculative and lacking adequate reasoning.

Determinative passage · p.3The WCJ's opinion found that apportionment was not appropriately addressed per the standards outlined in Escobedo v. Marshalls (2005) 70 Cal.Comp.Cases 604, 611 (en banc) because the physician did not clearly outline the pre-existing conditions nor did he adequately address with particularity how those conditions caused the current impairment. He accepted the QME's opinion that the impairment to the right and left shoulder should be added instead of combined. The WCJ also found that the correct occupational code is 330 based on the job description provided (Defendant's Exhibit A.)

The WCJ applied these standards to reject the QME's apportionment opinion for lack of adequate explanation and to rate permanent disability without apportionment.

Determinative passage · p.10Marshalls (2005) 70 Cal.Comp.Cases 604, 620-621 (Appeals Bd. en banc).) "Medical reports and opinions are not substantial evidence if they are known to be erroneous, or if they are based on facts no longer germane, on inadequate medical histories and examinations, or on incorrect legal theories. Medical opinion also fails to support the Board's findings if it is based on surmise, speculation, conjecture or guess." (Hegglin v. Workmen's Comp. Appeals Bd. (1971) 4 Cal.3d 162, 169 [36 Cal.Comp.Cases 93].) Section 4663 provides that "[a]pportionment of permanent disability shall be based on causation." (Lab. Code, § 4663(a).) A doctor who prepares a report addressing the issue of permanent disability due to a claimed industrial injury must address the issue of causation of the permanent disability. (Lab. Code, § 4663(b).) Section 4663 requires that the doctor "make an apportionment determination by finding what approximate percentage of the permanent disability was caused by the direct result of injury arising out of and occurring in the course of employment

The panel reviewed whether the medical opinions met the substantial evidence standard for apportionment under section 4663.

Determinative passage · p.3II. In addition to the reasons stated by the WCA, we note the following observations. The WCA, in significant part, based the opinion in this matter upon the recent panel decision of Canto Shadoan v. City of San Diego. (91 Cal.Comp.Cases 352, 2025 Cal.Work.Comp. P.D. LEXIX 435 (writ den.).). On April 29, 2026, the Fourth District Court of Appeal denied review in Canto Shadoan with an unpublished opinion expressing agreement with the logic of the case. The WCA did not find apportionment based upon causation in this matter. Both of applicant's disabilities were rated using the exact same method contained in the AMA Guides, which was range of motion. Both AMEs in both cases found that applicant's impairment was 100% industrial in accordance with section 4663(e). Both disability awards are based upon a finding of 100% industrial causation.

The WCAB found that apportionment based on causation is precluded under section 4663(e) but apportionment under section 4664(b) by subtraction is allowed.

Determinative passage · p.16(c) In order for a physician's report to be considered complete on the issue of permanent disability, the report must include an apportionment determination. A physician shall make an apportionment determination by finding what approximate percentage of the permanent disability was caused by the direct result of injury arising out of and occurring in the course of employment and what approximate percentage of the permanent disability was caused by other factors both before and subsequent to the industrial injury, including prior industrial injuries.

Applied to require apportionment determination in medical reports.