specific activity, 1. the radioactivity of a radioisotope per unit mass of the element or compound, expressed in microcuries per millimole or disintegrations per second per milligram. 2. the relative radioactivity per unit mass, expressed as counts per minute per milligram. For example, the specific activity of potassium in the human body is the same as that in the environment or diet. Because potassium is associated chiefly with muscle tissue, a whole-body count of 40K, after administration of radioactive potassium, can be used to distinguish lean body mass from total body mass.
