Apportionment is the most consequential battleground in the California workers' compensation permanent disability phase. Labor Code sections 4663 and 4664 require that permanent disability awards reflect only the industrial portion of a disability, with the non-industrial share excluded from the employer's obligation. In practice, this means that a defense-retained QME or AME who attributes 50 percent of a back injury to the applicant's degenerative disc disease will cut the permanent disability award in half. Getting the apportionment right, or defeating it entirely, is the difference between a meaningful permanent disability award and a fraction of one.
The Statutory Framework
Labor Code section 4663 imposes apportionment based on direct medical causation: a physician must make a determination of the approximate percentage of the permanent disability that is due to the direct result of injury arising out of and occurring in the course of employment, and the approximate percentage of the permanent disability that is due to other factors, including prior industrial injuries. Section 4664 adds a separate apportionment basis for prior awards: if the employee has received a prior award of permanent disability, the employer is entitled to a presumption that the prior disability still exists at the time of the new industrial injury, unless the employee rebuts that presumption with evidence of rehabilitation or full recovery.
The critical limitation on both statutes is the substantial medical evidence standard. An apportionment opinion that is speculative, conclusory, or not based on clinical findings is not substantial medical evidence and cannot support a reduced award. The WCAB and California appellate courts have developed a body of case law defining what a legally adequate apportionment opinion must contain, and that body of law is applicant counsel's primary tool for defeating inadequate defense apportionment.
What Makes an Apportionment Opinion Legally Adequate
An apportionment opinion must do more than cite a pre-existing condition and assign a percentage. The physician must explain the medical basis for the apportionment finding in terms that are both comprehensible and clinically grounded. A QME opinion that says the applicant has degenerative disc disease, which I apportion at 40 percent, without explaining why the degenerative changes are causally related to the current permanent disability, as opposed to being an incidental radiographic finding, is not substantial medical evidence.
The WCAB has held that apportionment is not appropriate for a condition that was asymptomatic and not causing functional impairment before the industrial injury. The mere presence of a pre-existing degenerative condition on imaging does not establish that the condition was causing the kind of disability that can be apportioned. This is the lighting-up doctrine: when an industrial injury causes a previously asymptomatic condition to become symptomatic and functionally limiting, the resulting disability may be entirely industrial in origin even if the underlying structural condition predated the injury.
Applicant counsel confronting a defense apportionment opinion should look for the following deficiencies: the opinion does not address whether the pre-existing condition was symptomatic before the industrial injury; the opinion does not explain how the physician arrived at the specific percentage; the opinion relies on radiographic findings without addressing functional status; or the opinion apportions to a condition that the medical record shows was not causing the applicant any work limitation or treatment before the industrial injury.
Section 4664 and the Prior Award Presumption
Section 4664's prior award presumption creates a procedurally distinct challenge. When the employer produces a prior award or stipulation establishing permanent disability, the presumption arises that the prior disability persists. The applicant must rebut that presumption with medical evidence showing rehabilitation or recovery. The rebuttal typically requires QME or AME testimony addressing the applicant's functional status since the prior award, any intervening treatment, and the clinical basis for concluding that the prior disability has resolved or improved to a degree that eliminates apportionment.
Applicant counsel should develop the rebuttal record early in the QME/AME evaluation process, not as an afterthought at trial. The treating physician's records in the period between the prior award and the current injury are the primary evidence base. If the applicant returned to full-duty work without restrictions after the prior award and maintained that status until the new injury, that work history is itself evidence of functional recovery. However, work history alone is generally insufficient without supporting medical evidence addressing the pre-existing condition's status.
Vocational Evidence and Apportionment
Apportionment affects the permanent disability rating, but vocational evidence can operate on a separate track in certain cases. Under the Ogilvie doctrine, an applicant can rebut the scheduled permanent disability rating with vocational rehabilitation evidence showing a greater actual diminished future earning capacity than the schedule reflects. When apportionment reduces the scheduled rating to a level that fails to capture the applicant's real-world earning loss, a vocational expert opinion can support a higher award on the rebuttable basis that the industrial injury caused functional limitations that the reduced scheduled rating does not reflect.
This is not a workaround for deficient medical evidence on apportionment; the Ogilvie rebuttal operates on the PD rating, not on the apportionment determination itself. But in cases where the defense has obtained an apportionment that appears legally sufficient and is unlikely to be reversed at the WCAB, the vocational track may recover part of what apportionment takes away from the permanent disability award. For the broader context on permanent disability strategy in California workers' comp, the workers-comp category covers the full range of QME/AME, apportionment, and PD rating issues. How permanent disability awards interact with third-party settlement recoveries and credit issues is addressed in the liens-and-settlement category. And for recent WCAB decisions on apportionment that bear on current case strategy, the case-law-settlements archive tracks the evolving appellate framework.