Under the AMA Guides, apportionment is based on what primary factor?

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

Answer: (A)

The thing being tested is how apportionment is determined under the AMA Guides: it relies on causation. Specifically, we ask what portion of the impairment is actually caused by the work‑related injury versus factors that existed before the injury, or other non-work factors. The key question is whether the impairment would exist if the injury hadn’t occurred. If removing the injury would leave the impairment the same, then that portion isn’t attributed to the injury; if removing the injury would reduce or eliminate the impairment, that portion is apportioned to the injury.

The treating physician’s report can inform the assessment, and pre-existing disability is a factor to consider, but they aren’t the primary basis for apportionment. An informal rating isn’t used for determining apportionment.