Panel reactive antibody
Panel Reactive Antibody (PRA) is an immunological laboratory test routinely performed on the blood of people awaiting organ transplantation. The PRA score is expressed as a percentage between 0% and 99%. It represents the proportion of the population to which the person being tested will react via pre-existing antibodies. These antibodies target the Human Leukocyte Antigen (HLA), a protein found on most cells of the body. Each population will have a different demographic of HLA antigens, and so the PRA test will differ from country to country.
A high PRA usually means that the individual is primed to react immunologically against a large proportion of the population. Individuals with a high PRA 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. These antibodies develop following previous transplants, blood transfusions and pregnancy. Transplanting organs into recipients who are "sensitized" to the organs significantly increases the risk of rejection, resulting in higher immunosuppressant requirement and shorter transplant survival. People with high PRA therefore spend longer waiting for an organ to which they have no pre-existing antibodies.
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.