Under AMA Guides apportionment is based on?

• A. Causation
• B. Medical report of treating physician
• C. Pre-existing disability
• D. Informal rating

Answer: (A)

Apportionment under the AMA Guides is driven by causation—the question is how much of the impairment is caused by the work-related injury versus other factors. The process uses a formal impairment rating from the AMA Guides, then allocates that impairment according to the causal contribution of the industrial injury. If the injury is entirely responsible, the full rating is attributable to the work injury; if a pre-existing condition also contributes, only the portion caused by the work injury is counted for apportionment. Medical reports help establish causation, but the fundamental basis for apportionment is determining the causal share of impairment, not simply referencing pre-existing disability or relying on an informal rating.