In general, impairment is a decreased function in a body part(s) or organ(s) established by medical evidence. Impairment ratings are based on the evaluation of specific medical tests (i.e. pulmonary function tests, liver function tests, etc.) that can be done by any qualified physician. In other words, medical evaluations necessary to evaluate the degree of loss of function of body parts that are affected by the covered condition.

Source: U.S. Department of Labor
http://www.dol.gov