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Crossmatching (also known as compatibility testing) establishes compatibility or incompatibility of a donor's and a recipient's blood. It's the best antibody detection test available for avoiding lethal transfusion reactions. After the donor's and the recipient's ABO and Rh-factor type are determined, major crossmatching determines compatibility between the donor's red blood cells (RBCs) and the recipient's serum. Minor crossmatching determines compatibility between the donor's serum and the recipient's RBCs. Because the antibody-screening test is routinely performed on all blood donors, minor crossmatching is often omitted.

Because a complete crossmatch may take from 45 minutes to 2 hours, an incomplete (10-minute) crossmatch may be performed in an emergency, such as severe blood loss due to trauma. In an emergency, transfusion can begin with limited amounts of group O packed RBCs while crossmatching is completed. Incomplete typing and crossmatching increase the risk of complications.

After crossmatching, compatible units of blood are labeled and a compatibility record is completed.

Clinical Alert: The most carefully performed crossmatch may not detect all the possible sources of patient-donor incompatibility.


  • To serve as the final check for compatibility between a donor's and a recipient's blood

Patient preparation

  • Explain to the patient that this test ensures that the blood he receives matches his own to prevent a transfusion reaction.
  • Inform him that he needn't fast before the test.
  • Tell him that the test requires a blood sample and who will perform the venipuncture and when.
  • Reassure him that although he may experience transient discomfort from the needle puncture and the tourniquet, collecting the sample takes only a few minutes.
  • Check the patient's history for recent administration of blood, dextran, or I.V. contrast media.

Procedure and posttest care

  • Perform a venipuncture, and collect the sample in a 10-ml red-top tube. ABO typing, Rh typing, and crossmatching are all done together.
  • If a hematoma develops at the venipuncture site, apply warm soaks.


  • Handle the sample gently to prevent hemolysis, which can mask hemolysis of the donor's RBCs.
  • Label the sample with the patient's name, the hospital or blood bank number, the date, and the phlebotomist's initials.
  • Indicate on the laboratory slip the amount and type of blood component needed.
  • Send the sample to the laboratory immediately.
  • If more than 72 hours have elapsed since an earlier transfusion, previously crossmatched donor blood must be recrossmatched with a new recipient serum sample to detect newly acquired incompatibilities before transfusion.
  • If the patient is scheduled for surgery and has received blood during the past 3 months, his blood needs to be crossmatched again if his surgery is rescheduled to detect recently acquired incompatibilities.

Normal findings

Absence of agglutination indicates compatibility between the donor's and the recipient's blood, which means that the transfusion of donor blood can proceed. Note that this doesn't guarantee a safe transfusion.

Abnormal findings

A positive crossmatch indicates incompatibility between the donor's blood and the recipient's blood, which means that the donor's blood can't be transfused to the recipient. The sign of a positive crossmatch is agglutination, or clumping, when the donor's RBCs and the recipient's serum are correctly mixed and incubated. Agglutination indicates an undesirable antigen-antibody reaction. The donor's blood must be withheld and the crossmatch continued to determine the cause of the incompatibility and identify the antibody.

Interfering factors

  • Recent administration of dextran or I.V. contrast media (causing cellular aggregation resembling antibody­mediated agglutination)
  • Previous blood transfusion (possibility of new antibodies to donor blood)
  • Hemolysis due to rough handling of the sample
  • Delay of testing for more than 72 hours after sample collection.

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