The percent PRA value is a measure of a patient’s level of sensitization to donor antigens. It is the percentage of cells from a panel of blood donors against which a potential recipient’s serum reacts. The PRA reflects the percentage of the general population that a potential recipient makes antibodies (is sensitized) against. The higher the PRA, the more sensitized a patient is to the general donor pool, and thus the more difficult it is to find a suitable donor. A patient may become sensitized as a result of pregnancy, a blood transfusion, or a previous transplant.