What Is California Labor Code §4663 (Permanent Disability Apportionment)?

(a) Apportionment of permanent disability shall be based on causation. (b) A physician who prepares a report addressing the issue of permanent disability due to a claimed industrial injury shall address in that report the issue of causation of the permanent disability. (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.

What does Labor Code 4663 actually allow?

Labor Code 4663 lets an insurer reduce the PD award by attributing part of the disability to non-work causes like prior injuries or degenerative conditions. The employer carries the entire burden of proof.

Labor Code 4663 is California's apportionment rule. It gives insurers a tool to reduce PD awards. They can attribute part of the disability to prior injuries, degenerative conditions, or other non-work factors. If they succeed, the award shrinks by that percentage. The worker does not have to disprove apportionment. The employer must prove every claim.

Eman Yazdchi is a Certified Specialist in workers' compensation law, certified by the California Board of Legal Specialization, State Bar of California. He challenges unsupported apportionment findings on every PD file at Yazdchi Law.

How does apportionment under Labor Code 4663 reduce a PD award?

If a medical evaluator attributes 40% of a lumbar disability to pre-existing degeneration and 60% to the work injury, the employer pays only 60% of the full PD value under the Labor Code 4658 schedule.

Here is a concrete example. A worker has a PD value of $100,000. The evaluator assigns 40% to non-industrial causes. The insurer pays $60,000. The reduction is direct and mechanical. It applies dollar-for-dollar to the payment schedule under Labor Code 4658.

Who must prove apportionment under Labor Code 4663?

The employer bears the entire burden. They must produce a medical opinion naming a specific non-industrial cause and quantifying its contribution. An asymptomatic prior condition with no prior PD award is a weak basis.

The employer must present substantial medical evidence. A Panel QME under Labor Code 4062.2 or an AME must identify the exact non-industrial cause. The opinion must assign a percentage. Vague conclusions are not enough. WCAB judges routinely reject them.

What is the substantial-medical-evidence standard for apportionment?

The medical opinion must name the specific non-industrial cause, explain the mechanism, and quantify its share using the AMA Guides Fifth Edition framework. Conclusory percentage estimates are routinely rejected.

A conclusory opinion will not survive WCAB review. The evaluator must name the cause. They must explain the mechanism. They must support the percentage with AMA Guides reasoning. Opinions that merely state a percentage without analysis fail the substantial-evidence test.

How does Labor Code 4663 interact with Labor Code 4664?

Labor Code 4663 covers all non-industrial causes. Labor Code 4664 addresses prior permanent disability awards for the same body part, which are credited against the current rating dollar-for-dollar.

California has two related apportionment rules. Labor Code 4663 covers any non-industrial cause of disability. Labor Code 4664 covers prior PD awards for the same body region. When a worker had a prior award, that prior disability is subtracted from the current rating. Labor Code 4663 handles most apportionment disputes. Labor Code 4664 applies in prior-PD overlap cases.

Related: California settlement pillar · Labor Code 4660 explainer.

Eman Yazdchi is a Certified Specialist in workers' compensation law, certified by the California Board of Legal Specialization, State Bar of California. Yazdchi Law serves injured workers throughout Greater Los Angeles. We appear at the WCAB in Van Nuys, Los Angeles, Long Beach, Pomona, San Bernardino, Riverside, and Oxnard. Call (661) 273-1780 for a free case review.