It is an immunologic metric routinely performed by clinical laboratories on the blood of people awaiting organ transplantation.
[1] Traditionally serum is exposed to panel lymphocytes and to an extent other leukocytes in a complement dependent cytotoxicity test.
Individuals with a high PRA value are often termed "sensitized", which indicates that they have been exposed to "foreign" (or "non-self") proteins in the past and have developed antibodies to them.
[3] Extensive efforts have been made to identify treatment regimes to reduce PRA in sensitized transplant candidates.
In certain circumstances, plasma exchange, intravenous immunoglobulin, rituximab and other "antibody-directed" immune therapies may be employed, but this is an area in which active investigation continues.