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 in the AMA Guides is driven by causation: determine how much of the claimant’s permanent impairment is caused by the work-related injury versus preexisting factors. Medical evidence, including findings from the treating physician, informs the split, but the basis is the causal contribution supported by the AMA impairment criteria, not an informal rating or a single opinion alone. Pre-existing disability matters only to the extent it represents impairment not caused by the work injury; the work-related portion is the impairment attributable to the industrial event. For example, if total impairment is 20% and evidence shows 8% is caused by the work injury while 12% stem from a preexisting condition, only the 8% is attributed to the industrial accident.