This term (abbreviated “PRA”) is both a test and a result! The PRA test was commonly used in HLA testing laboratories in the past, and the same term was used for the result of the test. The PRA measured the incompatibility of serum from a person or patient against a panel of non-self white blood cells. The test was done one of several ways, but the PRA was designed to detect the presence of antibodies against HLA antigens. The percentage of the test cells against which the serum reacted was also called the “PRA” (technically, the PRA “score,” but pretty much everyone just said “PRA”), and generally represented the percentage of the general population against which the patient was incompatible (e.g., a PRA of 75 means that the person had antibodies against 75% of the population). I’m sure you noticed that all of the preceding sentences were written in past tense! The old PRA method has been replaced by sensitive antibody specificity testing so that HLA laboratories can identify specific antibodies. The antibody specificities can then be used to generate a “calculated PRA” (cPRA), which has the same basic meaning as the old PRA score. The cPRA may be useful to blood bankers in the management of patients with platelet refractoriness (a situation where platelet transfusions lead to an insufficient response in the patient), because it helps to predict the percentage of random donors who are expected to be incompatible with the patient’s anti-HLA antibodies.