Under the AMA Guides apportionment, which factor is used to determine apportionment?

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

Answer: (A)

Apportionment is determined by causation—the question asks which portion of impairment is caused by the work-related injury versus non-work factors. The AMA Guides require separating impairment into parts attributable to industrial factors and to non-industrial factors (like a pre-existing disability). If the impairment is entirely due to non-work factors, there’s no apportionment to the work injury; if it’s partly due to work, the appropriate portion is attributed to the industrial factor. The medical report helps establish causation, but the key determinant is the causal relationship itself, not the treating physician’s report or an informal rating.