Apportionment is the process of separating impairment caused by the current injury from impairment that existed before the injury occurred. When a patient has pre-existing conditions that affect the same body region as the new injury, the physician must determine how much of the current impairment is attributable to the injury in question, and how much was already present. Only the portion attributable to the current injury is compensable.
Apportionment subtracts the impairment that was already present from the current impairment rating, so the patient is compensated only for the portion caused by the injury at issue.
Why Apportionment Exists
The underlying principle is straightforward: a workers' compensation claim, personal injury case, or disability determination should compensate the patient for the harm caused by the specific event or exposure. A patient who had a 10% cervical spine impairment before a workplace injury should not receive full compensation for a post-injury rating of 20% if the pre-existing 10% is unrelated to the workplace.
Apportionment protects the integrity of the system for employers, insurers, and patients alike. A physician who fails to apportion when pre-existing conditions clearly exist is producing an inflated rating. A physician who apportions without adequate documentation is producing a deflated one. Both are errors.
What Qualifies as a Pre-Existing Condition for Apportionment?
The AMA Guides address apportionment, but jurisdictions vary significantly in what they require. In Nevada, NRS 616C.099 governs apportionment and is more restrictive than the Guides alone. Under Nevada law, apportionment must be based on documented prior conditions, specifically:
- Prior medical records predating the injury that document the condition
- A prior impairment rating from any state or edition of the AMA Guides
- Physical evidence of prior surgery visible on examination or imaging
A patient's self-report that they had a prior condition is not sufficient basis for apportionment under Nevada law. If no documentation exists, the physician cannot apportion, but should note in the report that records indicating a prior condition were not available, and that the rating would be revised if such records were provided.
How to Apply Apportionment Correctly
The specific methodology for apportionment depends on the applicable edition of the AMA Guides and the jurisdiction. In Nevada, the percentage from the prior rating is used directly against the current rating; no conversion between editions is required. If the prior rating used the 4th Edition on the lumbar spine and the current rating uses the 5th Edition on the same region, the prior percentage applies as-is.
The report must clearly document: the basis for the apportionment, the source of the prior rating or medical evidence, the specific percentage being apportioned, and the net WPI after apportionment. Vague apportionment language such as "I am apportioning some percentage to pre-existing conditions" is not adequate documentation.
When You Cannot Find the Prior Records
If a patient reports a prior injury but no records are available, document that the claim of a prior condition exists but cannot be confirmed. Do not apportion without documentation. Note in the report that if records substantiating the prior condition are provided, the rating may be revised. This protects you, keeps the report accurate, and puts the burden of producing documentation where it belongs.
Frequently Asked Questions
Does apportionment apply in all states?
Apportionment principles exist across most workers' compensation systems, but the specific rules vary significantly by jurisdiction. Nevada's apportionment rules under NRS 616C.099 are more restrictive than the AMA Guides alone. Always apply the jurisdiction's rules, not just the Guides.
Can I apportion based on imaging findings alone?
It depends on the jurisdiction and the specific finding. Physical evidence of prior surgery, such as a surgical scar confirmed on examination or hardware visible on imaging, can support apportionment in Nevada even without prior records. Degenerative findings on imaging that predate the injury are more nuanced and require careful documentation of their relationship to the injury.
What if the patient disputes the apportionment?
Document the clinical and record basis for the apportionment clearly and specifically. A well-documented apportionment with clear citations to the applicable statute, edition of the Guides, and supporting records is far more defensible than one that relies on the physician's general clinical judgment.
CIRS™ certification through AAEME covers apportionment methodology, AMA Guides application, and Nevada-specific rules, with CME/CEU credit for MDs, DOs, and DCs.
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