Reproduced from the AMA Guides 5th edition, page 11, 1.6B

Apportionment analysis in workers’ compensation represents a distribution or allocation of causation among multiple factors that caused or significantly contributed to the injury or disease and resulting impairment. The factor could be a preexisting injury,

illness, or impairment. In some instances, the physician may be asked to apportion or distribute a permanent impairment rating between the impact of the current injury and the prior impairment rating. Before determining apportionment, the physician needs to

verify that all the following information is true for an individual:

  1. There is documentation of a prior factor.
  2. The current permanent impairment is greater as a result of the prior factor (ie, prior impairment, prior injury, or illness).
  3. There is evidence indicating the prior factor caused or contributed to the impairment, based on a reasonable probability (> 50% likelihood).

The apportionment analysis must consider the nature of the impairment and its possible relationship to each alleged factor, and it must provide an explanation of the medical basis for all conclusions and opinions. Most states have their own customized methods for calculating apportionment. Generally, the most recent permanent impairment rating is calculated, and then the prior impairment rating is calculated and deducted. The remaining impairment rating would be attributed or apportioned to the current injury or condition.


A common verbal formulation in the workers’ compensation context might state, “in cases of permanent disability less than total, if the degree of disability resulting from an industrial injury or occupational disease is increased or prolonged because of a preexisting physical impairment, the employer shall be liable only for the additional disability from the injury or occupational disease.”


For example, in apportioning a spine impairment rating in an individual with a history of a spine condition, one should calculate the current spine impairment. Then calculate the impairment from any preexisting spine problem. The preexisting impairment rating is then subtracted from the present impairment rating to account for the effects of the former. This approach requires accurate and comparable
data for both impairments.