Antibody Screen and Percent Reactive Antibody Assay

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The page below is a sample from the LabCE course The Human Leukocyte Antigen (HLA) System. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Antibody Screen and Percent Reactive Antibody Assay

  • Serum can be treated to enhance reactivity and reduce interfering agents.
    • EDTA and DTT are common molecules added to treat/enhance antibody screening techniques.
  • HLA laboratories regularly screen patients for the development of new or existing antibodies to determine the percent reactive antibody (PRA) for allocation of organs to transplant.
    • Sensitization with HLA antibodies typically results from exposure to foreign antigens by blood transfusion, previous transplantations, or previous pregnancy (paternal antigens on fetal cells).
    • This is similar to Landsteiner's rule in blood banking ("You make antibody against an antigen that you lack.")
  • Antibody screening is performed by adding patient serum to known antigens that are fixed to a medium, like latex bead particles. After washing to remove any unbound, excess antibodies, anti-human globulin (AHG) bound with a fluorescent marker is added for detection.
  • For detection, most labs utilize some solid phase technology or ELISA.
  • PRA assesses a recipient's serum for HLA antibodies to determine the percentage of potential donors the patient could react to if transplanted with a donor.
    • Example: A calculated PRA of 40% would imply that the patient has a 40% chance of mounting an immune response, or rejecting, organs of donors in the given population. The lower the PRA, the better the chance of finding an organ that would be ideal for transplantation.
  • PRA percentages vary per lab and are established by comparing HLA loci of populations of donors and recipients living in various geographic areas.