Writing an Apportionment Section That Withstands Scrutiny
In California workers’ compensation, few parts of a medical-legal report invite more scrutiny than apportionment. The apportionment section is where credibility is tested, disputes begin, and outcomes are decided. Judges, attorneys, and DEU raters depend on this section to determine what portion of permanent disability stems from industrial versus non-industrial causes. When written clearly, apportionment is the key to resolving a claim. This guide explains how to make your apportionment section both defensible and persuasive.
Understand the Legal Standard
According to Labor Code §§ 4663 and 4664, apportionment is not about dividing an injury or injuries, it is about dividing permanent disability. A complete opinion must determine the approximate percentage of permanent disability caused by the industrial injury, and the approximate percentage caused by other factors that occurred before or after that injury, whether industrial or non-industrial.
Under Escobedo v. Marshalls (2005) 70 Cal. Comp. Cases 604 (En Banc), a valid apportionment opinion must explain how and why the physician arrived at each percentage. Specifically, how the non-industrial factors did or did not contribute to permanent disability and why those causes are medically reasonable.
Your opinion must be framed in reasonable medical probability, supported by an accurate history and examination, and must clearly explain the rationale supporting each conclusion.
Build Your Opinion on Solid Evidence
A defensible apportionment opinion rests on facts, not assumptions. Anchor your percentages in objective findings, e.g. diagnostic studies confirming pre-existing or degenerative pathology, clinical evidence of structural or functional impairment, or historical documentation of prior injuries, illnesses, or treatment.
Example: “The applicant’s multilevel lumbar degenerative disc disease, confirmed by pre-injury MRI, accounts for approximately 40% of the current disability.”
Connect the Dots Between Pathology and Function
Naming a condition is not enough, you must demonstrate its functional contribution to the current permanent disability. How does the condition limit the worker today? The issue is really about activities of daily living. Why and how does it continue to contribute to the applicant’s maximum medical improvement or permanent and stationary status?
Example: “While the industrial injury produced an acute disc herniation, longstanding L4–L5 degenerative changes pre-dating the DOI continue to restrict motion and contribute to pain.”
It is important to distinguish between pre-existing pathology and pre-existing disability. Apportionment may be based on pre-existing pathology that is causing disability, even if it was asymptomatic before injury, provided the physician explains how it contributes to current permanent impairment.
Justify the Percentage
Avoid arbitrary figures like 50/50. A percentage should emerge from your medical reasoning and analysis of objective evidence. Substantiate your conclusion by comparing baseline to post-injury function, describing the progression of the pathology over time, and referencing medical literature.
Example: “Given the applicant’s pre-existing AC joint arthritis and documented shoulder impingement treatment, I apportion approximately 25% of the current permanent disability to non-industrial causes.”
Your rationale should align with the Escobedo “how and why” framework and meet DWC Quality Assurance expectations for clarity and medical reasoning.
Use Language That Withstands Scrutiny
Your word choice signals whether your opinion meets legal and evidentiary standards. Use defensible phrases such as:
- “Based on reasonable medical probability”
- “Supported by objective medical evidence”
- “Reflects the pathological contribution to current disability”
Avoid vague or discriminatory statements. Apportioning to age, gender, or obesity without evidence of an actual disabling condition can be problematic and may invalidate your opinion.
Causation of Injury versus Causation of Disability
Do not confuse causation of injury with causation of disability. This is one of the most common errors in QME reporting. The causation of injury refers to the industrial event that produced harm (for example, lifting a heavy box and sustaining a lumbar disc herniation). The causation of disability refers to what portion of the worker’s disability is due to the industrial injury versus other factors such as pre-existing degenerative disease.
Example: “The applicant’s acute L5–S1 disc herniation was entirely caused by the 3/12/2024 industrial lifting injury (100% industrial). However, MRI evidence of pre-existing multilevel lumbar degeneration continues to contribute to the worker’s ongoing limitation and pain. Based on reasonable medical probability, 70% of the current lumbar permanent disability is attributable to the industrial injury, and 30% to the pre-existing non-industrial degenerative disease.”
In other words, the injury itself can be 100% industrial, while the resulting disability can still be apportioned between industrial and non-industrial factors.
Avoiding Pitfalls
- Do not apportion risk instead of cause. Risk factors are not disabling conditions.
- Do not assign apportionment to temporary disability. Only permanent disability is subject to apportionment.
- Do not aggregate multiple non-industrial factors into a single undifferentiated percentage. Each factor should be described with its own reasoning.
- Do not fail to explain when apportionment is not possible. If disability cannot be reasonably apportioned, state so explicitly and explain why.
Close with Clarity and Confidence
Your final paragraph should read like a legal finding. It should be concise, transparent, and grounded in evidence.
Example Conclusion: “Within reasonable medical probability, 60 percent of the applicant’s lumbar permanent disability is attributable to the industrial injury of 4/3/22, and 40 percent to pre-existing degenerative disease documented by MRI prior to that date. This opinion is supported by objective findings and consistent with Labor Code §§ 4663 and 4664 and the Escobedo ‘how and why’ standard. No other non-industrial factors are identified that contribute to the applicant’s present level of permanent disability.”
Final Thoughts
Apportionment is not a bureaucratic box to check, it is a legal opinion built on medical reasoning. Done well, apportionment clarifies complex causation and protects the integrity of your report.
By explaining what is apportioned, how it was determined, and why it makes medical and legal sense, you produce an opinion that satisfies both the Escobedo “how and why” requirement and the Division of Workers’ Compensation Quality Assurance criteria.
Be clear. Be precise. Be persuasive. That is how you make apportionment count.
Want to make sure your reports are written to meet quality standards? Download the DWC Quality Assurance Checklist to guide your next evaluation: https://share.google/X1i7VHXpMNDvIBKcs
Want to learn more? Read our post on building a defensible medical-legal report: https://simplexam.com/2025/10/24/report-quality-how-to-build-a-defensible-med-legal-report/