Reproduced from the AMA Guides 6th edition, page 25, 2.5c

Apportionment is an allocation of causation among multiple factors that caused or significantly contributed to the injury or disease and resulting impairment. Apportionment requires a determination of percentage of impairment directly attributable to preexisting as compared with resulting conditions and directly contributing to the total impairment rating derived. In such cases the rating physician may estimate these contributions by first developing the following contingent ratings as based on earlier work:

  1. A “total” impairment rating (A) (an all-inclusive current rating) is derived irrespective of preexisting and resulting conditions.
  2. A second “baseline” rating (B) is derived that accounts solely for preexisting conditions without associated or aggravating reinjury.
  3. The final rating (C) is derived in which preexisting conditions are discounted by subtracting the second from the first rating (A – B).

If apportionment is needed, the analysis must consider the nature of the impairment and its relationship to each alleged causative factor, along with an explanation of the medical basis for all conclusions and opinions. Using this approach to apportionment requires accurate information and data to determine all impairment ratings both before and after the most recent injury. If different editions of the Guides have been used, the physician must assess their similarity. If the basis of the ratings is similar, a subtraction is appropriate.
If the bases of the ratings differ markedly, the physician should evaluate the circumstances and determine whether conversion to the earlier or latest edition of the Guides for both ratings is possible. The determination should follow the local jurisdiction’s guidelines and
consider whichever edition best describes the individual’s impairment. If no rating was previously assigned, the examiner must use available information to estimate what the rating was before the new injury, and subtract this from the “new” rating as noted earlier.