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 determined by causation—the portion of a worker’s impairment that is caused by the work-related factors versus non-industrial factors (like a preexisting condition). The idea is to separate impairment according to its causal source: what portion would not have occurred but for the industrial event belongs to the work-related apportionment, while the rest remains due to non-industrial factors. A treating physician’s report can inform the causal assessment, but the fundamental basis is determining how much impairment is caused by the work factor. Pre-existing disability matters as the baseline but does not itself set the apportionment; informal ratings are just a method of rating, not the basis for apportionment.