Apportionment in California Workers' Comp: Causation of Injury vs. Causation of Disability

Q: Does a QME have to use exact percentages for apportionment?

No. Labor Code section 4663 asks for approximate percentages, and stating them as approximate is standard practice.

Short answer: apportionment divides permanent disability between the industrial injury and everything else. Under Labor Code §4664(a) the employer is liable only for the percentage of permanent disability directly caused by the work injury, and under §4663 the physician must state what approximate percentage was caused by the injury and what approximate percentage by other factors, before and after, including prior industrial injuries. The single most common way a QME's apportionment opinion fails is apportioning the causation of the injury rather than the causation of the disability. Those are different questions, and an opinion that confuses them is not substantial evidence.

Every experienced evaluator says the same thing about this: apportionment is the hardest determination in the job. Continuing education conferences give it a third of the agenda for a reason.

What is apportionment in workers' comp?

Before 2004, an employer was generally on the hook for the whole disability, with narrow exceptions. Senate Bill 899 changed that. Labor Code §§4663 and 4664, enacted as part of that reform, limit the employer to the share of permanent disability the industrial injury actually caused.

The practical effect is that you are telling the employer how much they owe and telling the injured worker how much of their own disability they have to absorb. Both sides read that section closely, and both will scrutinize the percentages.

Two thresholds gate the analysis. Apportionment only applies when the examinee is permanent and stationary, and only when there is a permanent impairment to apportion. No P&S, no apportionment. No impairment, nothing to divide.

An impairment opinion without an apportionment determination is incomplete on the issue of permanent disability and cannot resolve the dispute, which means a supplemental report and a delayed case.

The distinction that decides most opinions

Physicians may not apportion the causation of the injury. They apportion the causation of the disability.

Here is why that matters. Suppose an examinee has a transitional vertebra in the lumbar spine, a congenital anomaly. They bend to pick something up at work and injure their back. The anomaly plausibly contributed to the injury: a minimal mechanism produced a real injury partly because the spine was already unusual.

That is causation of injury. It is not, by itself, apportionment.

The apportionment question comes later, once the worker has treated, failed to fully recover, and is left with permanent impairment. Now the question is what caused that impairment. The anomaly may be contributing to it. It may not. Those are separate determinations, and an opinion that slides from one to the other is defective.

The Appeals Board and the courts treat this as a recurring error, particularly in cumulative trauma cases and in events like heart attacks and strokes where the line between the injury and the resulting disability is blurry. City of Jackson v. WCAB (Rice) (2017) 11 Cal.App.5th 109 is the case usually cited when the distinction comes up.

One useful consequence: how you characterize the disability can determine whether a pre-existing condition contributes to it. If you describe the impairment as a back disability, a pre-existing lumbar anomaly is likely contributing. If, using the four corners of the AMA Guides, you find the impairment is most accurately captured as a gait derangement, the anomaly may have nothing to do with it. That is not gamesmanship; it is why the characterization has to be accurate and reasoned.

What §4663 actually asks for

The statute requires a determination of what approximate percentage of the permanent disability was directly caused by the industrial injury and what approximate percentage was caused by other factors, both before and after the injury, "including prior industrial injuries."

Two words carry weight.

Approximate. The Legislature knew nobody can distinguish 54 percent from 51 percent. Approximate percentages are what the statute asks for, and nobody will challenge you for using them. Write "approximately 50 percent," not "50 percent."

Other factors is otherwise undefined, with one example given: prior industrial injuries. That example is deliberate. Legislators knew that injured workers are often repeat injured workers, and that charging one employer for damage done at a previous job was a driver of cost. The statute does not list prior sports injuries, prior car accidents, or prior falls, but nothing excludes them either, provided they are established in the record.

Where the risk-factor line sits

This is the part that has moved, and the part that gets QMEs disciplined when they get it wrong.

Apportionment to a risk factor is impermissible. A factor that raises the risk of a particular injury is not evidence that the factor caused any portion of the disability. Risk of injury is not the same as cause of disability. The DWC's own QME study guide is explicit about this, and it is explicit about the reason: apportioning to gender, age, or another protected characteristic reduces a worker's compensation on a basis the law does not permit.

The example the DWC uses is carpal tunnel syndrome. An evaluator who apportions 50 percent of a woman's carpal tunnel impairment to the fact that women develop carpal tunnel more often than men has apportioned to gender. That is discrimination, not a medical determination.

Apportionment to a documented condition is permissible, even when the condition correlates with age or gender. The same evaluator, finding prior episodes of carpal tunnel syndrome documented in the examinee's own medical history, may apportion to that condition. Apportioning to age is impermissible; apportioning to pre-existing arthritis established in the records is permissible, even though arthritis is age-related. Apportioning to menopause as a risk factor is impermissible; apportioning to documented pre-existing non-industrial carpal tunnel syndrome is permissible.

The line is between a statistical generalization about a category of people and medical evidence about this person.

And then there is City of Jackson. In that case a QME apportioned 49 percent of a young police officer's cervical disability to his personal history, including heritability and genetics, based on degenerative disc disease found on imaging. The Appeals Board reversed. The Third District Court of Appeal, in a published 2017 decision, annulled the Board and held that apportionment to pre-existing genetic factors is permissible.

The two lines are not obviously reconcilable, and practitioners argue about it. The workable reading is that the individual's own documented pathology can be apportioned to, whatever its origin, while a generalization about a demographic group cannot. An evaluator relying on City of Jackson needs actual evidence about this examinee's condition, not a statement that a condition is common in a population. Anti-bias training is now part of QME education for exactly this reason.

Writing an apportionment opinion that survives

Because apportionment is scrutinized harder than anything else in the report, the criteria for substantial medical evidence apply with full force. Under Escobedo v. Marshalls (2005) 70 Cal.Comp.Cases 604, an apportionment opinion must disclose familiarity with the concepts of apportionment, describe in detail the exact nature of the apportionable disability, and set out the basis for the opinion so the Board can determine whether the physician is applying correct legal principles.

In practice, the section has a shape:

State the impairment precisely. Not "spine impairment" but the specific rating, the chapter and method it came from, and what it describes. You cannot apportion something you have not defined.
Frame the opinion in reasonable medical probability. "Under Labor Code §4663, in my opinion and within reasonable medical probability, approximately X percent of the permanent impairment is due to the industrial injury and approximately Y percent to other factors."
Name the other factors. Specifically. The 2019 MRI showing pre-existing degenerative change. The 2016 claim for the same body part. The documented prior condition.
Give the reasons. "My reasons for this conclusion include:" followed by the pertinent facts, each tied to something in the record or the examination.
Confirm the foundation. Adequate history, adequate examination, review of the studies, no speculation.

If the percentages come out at zero and one hundred, say so, still with "approximately," and still with reasons. A clean, fully reasoned zero-percent-industrial opinion is more useful to the system than a hedged split nobody can follow.

Why the records decide this section

Every legitimate basis for apportionment lives in the medical record. The prior industrial claim. The pre-injury imaging. The primary care note from four years earlier describing the same complaint. The old work restriction.

An evaluator who has not seen those pages cannot apportion to them, and an evaluator who apportions without them is speculating, which fails Escobedo on its own terms. That is the practical reason apportionment opinions get overturned: not bad judgment, but a thin factual base.

Lexamed reads the full production and builds a dated chronology with a page citation on every entry, flagging prior injuries to the same body part, pre-existing findings, and earlier complaints. When the reasons paragraph says the examinee reported the same knee pain to a treating physician eighteen months before the industrial injury, it points to the page it came from.

Frequently asked questions

What is apportionment in workers' compensation? Apportionment divides permanent disability between the industrial injury and other causes. Under Labor Code §4664(a), the employer is liable only for the percentage of permanent disability directly caused by the work injury.

What is the difference between causation of injury and causation of disability? Causation of injury asks what caused the injury to happen. Causation of disability asks what caused the permanent impairment that remains afterward. Apportionment addresses only the second. A factor that contributed to the injury may or may not contribute to the disability, and treating them as the same question makes the opinion invalid.

Can a QME apportion to age, gender, or genetics? Not to age or gender as risk factors; doing so is impermissible discrimination. Apportionment to a documented medical condition is permissible even where that condition correlates with age or gender. In City of Jackson v. WCAB (Rice) (2017), the Court of Appeal held that apportionment to pre-existing genetic factors was permissible where supported by medical evidence about that individual.

Does a QME have to use exact percentages for apportionment? No. Labor Code §4663 asks for approximate percentages, and stating them as approximate is standard practice.

When does apportionment apply? Only when the examinee is permanent and stationary and has a permanent impairment. If there is no P&S status or no impairment, there is nothing to apportion.

What happens if a QME report omits apportionment? A report that gives a permanent impairment opinion without an apportionment determination is incomplete on the issue of permanent disability and cannot resolve the dispute, which typically means a supplemental report is requested.

What makes an apportionment opinion substantial medical evidence? Under Escobedo, the opinion must disclose familiarity with apportionment, describe in detail the exact nature of the apportionable disability, and set out the basis and reasoning so the Board can determine whether correct legal principles were applied. It must also rest on reasonable medical probability, pertinent facts, and an adequate history and examination.