In this guide8 sections
Labor Code §4663 lets the insurance company reduce your permanent disability (PD) award when part of your disability was caused by something other than your work injury, such as an earlier injury or a pre-existing condition. This is called apportionment, and under §4663 it must be based on causation. The evaluating doctor must estimate what percentage of your permanent disability the work injury caused and what percentage other factors caused. Because apportionment can cut a PD award sharply, the doctor’s opinion must be based on solid medical reasoning, and the insurer bears the burden of proving it.
Key takeaways
- Apportionment of permanent disability “shall be based on causation” (§4663(a)); your employer is liable only for the percentage of PD directly caused by the work injury (§4664(a)).
- A doctor’s PD report isn’t complete without an apportionment determination, or a specific explanation of why one can’t be made (§4663(c)).
- You must disclose prior permanent disabilities or impairments if asked (§4663(d)).
- A prior PD award is conclusively presumed to still exist at the time of a later injury (§4664(b)).
- Apportionment opinions must be substantial medical evidence, and the insurer has the burden of proving apportionment.
What Labor Code §4663 says
- (a) Causation. Apportionment of permanent disability is based on causation, meaning what caused the disability, not only what caused the injury.
- (b) The doctor must address causation. Any physician who prepares a report on permanent disability must address the causation of that disability.
- (c) Apportionment percentages. To be complete, the report must estimate the approximate percentage of permanent disability caused directly by the work injury and the approximate percentage caused by other factors before and after the injury, including prior work injuries. If the doctor can’t make that determination, the report must state the specific reasons, and the doctor must consult with or refer you to another physician who can.
- (d) Disclosure. If asked, you must disclose all previous permanent disabilities or physical impairments.
- (e) Exceptions. Subdivisions (a) through (c) don’t apply to injuries covered by certain presumption statutes, Labor Code §§3212 to 3212.12, 3212.85, 3213 and 3213.2, which mainly cover public safety workers such as many police officers and firefighters.
How §4664 works with §4663
Labor Code §4664 adds three rules. First, your employer is liable only for the percentage of permanent disability directly caused by the work injury. Second, if you received a prior PD award, that disability is conclusively presumed to exist when the new injury happened, so it is subtracted. Third, PD awards for any one body region, such as the spine or the upper extremities, generally can’t exceed 100% over your lifetime, unless the injury is conclusively presumed total under §4662.
How apportionment affects your award: an example
Here is a hypothetical. Elena, a warehouse worker, injures her lower back lifting boxes. Ten years earlier she hurt her back in a car accident but never filed a claim and kept working full duty. After treatment, a QME rates her final permanent disability at 30% and finds that 60% is caused by the work injury and 40% by degeneration from the old car accident.
| No apportionment | 60% apportioned to work | |
|---|---|---|
| PD rating | 30% | 18% |
| Weeks of PD payments (injuries 2013 and later, §4658) | 131 | 65.5 |
| Total at the $290 weekly maximum | $37,990 | $18,995 |
Because PD payment weeks increase faster at higher ratings, a 40% apportionment here cuts the payout in half. For injuries on or after January 1, 2014, the PD weekly rate is between $160 and $290 depending on your earnings. These figures are illustrative only; estimate your own numbers with the workers’ comp benefits calculator or read what your PD rating is worth.
When apportionment can be challenged
Under a 2005 en banc decision of the Workers’ Compensation Appeals Board, an apportionment opinion must be framed in terms of reasonable medical probability, must not be speculative, and must explain the reasoning behind its conclusions. The insurer has the burden of proving apportionment. Common problems include:
- No explanation. The doctor names a percentage but doesn’t explain how and why the other factor is causing part of your current disability.
- Confusing injury with disability. An old condition that caused no symptoms or limits isn’t automatically a cause of today’s disability; the question is what caused the permanent disability.
- Bare reliance on age or imaging. Pointing to “degeneration” on an MRI, without connecting it to your current impairment, may not be enough.
- Incomplete records. The doctor may have misread or lacked the records of the prior injury.
If a report has these problems, your attorney may depose the doctor, ask for a supplemental report or argue that the report isn’t substantial evidence. See how to dispute a QME or AME report and our guide to pre-existing conditions in workers’ comp.
What you should do
- Answer disclosure questions honestly. Section 4663(d) requires you to disclose prior permanent disabilities or impairments if asked. Hiding them can damage your credibility.
- Describe your history accurately. Tell the doctor how you functioned before the work injury, such as full duty, no treatment and no restrictions.
- Gather prior records. Records showing you recovered from an earlier injury can rebut apportionment to it.
- Read the report. Check whether the apportionment section explains its reasoning.
- Consider repetitive-strain factors. If your job itself wore down a body part over time, that may be a work cause, not a non-industrial one; see cumulative trauma injuries.
Frequently asked questions
Can my PD be reduced because I’m older?
Not just because of your age. The doctor must explain with reasonable medical probability how a specific factor, such as documented degeneration, is causing part of your current disability.
Does apportionment affect my medical treatment?
Section 4663 governs apportionment of permanent disability. It doesn’t, by itself, reduce your right to medical treatment for the work injury.
I had a prior workers’ comp award to the same body part. What happens?
Under §4664(b), the prior PD award is conclusively presumed to still exist at the time of the new injury, so it is generally subtracted from the new rating.
Can the doctor refuse to apportion?
The doctor can say apportionment isn’t possible, but must give specific reasons and consult with or refer you to another physician who can make the determination (§4663(c)).
Talk to a California workers’ comp lawyer
Employees First Labor Law represents workers exclusively in workers’ comp cases across California. We offer free virtual consultations to review apportionment in a QME, AME or treating doctor’s report. Contact us or call (310) 853-3461. The DWC Information and Assistance Unit also gives free help to injured workers.
This article is general information about California law as of 2026, not legal advice for your situation. Reading it does not create an attorney-client relationship.
