Antibody Screening Test

Also called the indirect Coombs' test, the antibody screening test detects unexpected circulating antibodies in the patient's serum. After incubating the serum with group O red cells, which are unaffected by anti-A or anti-B anti­bodies, an antiglobulin (Coombs') serum is added. Agglutination occurs if the patient's serum contains an antibody to one or more antigens on the red cells.

The antibody screening test detects 95% to 99% of the circulating antibodies. After this screening procedure detects them, the antibody identification test can determine the specific identity of the antibodies present.

Purpose

• To detect unexpected circulating antibodies to red cell antigens in the recipient's or donor's serum before transfusion
• To determine the presence of anti-D antibody in maternal blood
• To evaluate the need for Rh immune globulin administration
• To aid diagnosis of acquired hemolytic anemia

Patient preparation

• Explain to the prospective blood recipient that the antibody screening test helps evaluate the possibility of a transfusion reaction.
• If the test is being performed because the patient is anemic, explain to him that it helps identify the specific type of anemia.
• Inform the patient 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 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-mi red-top or lavender-top tube. Some laboratories require 20 ml of clotted blood to perform this test.
• If a hematoma develops at the venipuncture site, apply warm soaks.

Precautions

• Handle the sample gently to prevent hemolysis.
• Label the sample with the patient's name, the hospital or blood bank number, the date, and the phlebotomist's initials. Be sure to include on the laboratory slip the patient's diagnosis and pregnancy status, any history of transfusions, and any current drug therapy.
• Send the sample to the laboratory immediately.

Normal findings

Normally, agglutination does not occur, indicating that the patient's serum contains no circulating antibodies other than anti-A or anti-B.

Abnormal findings

A positive result indicates the presence of unexpected circulating antibodies to red cell antigens. Such a reaction demonstrates donor and recipient incompatibility.

A positive result in a pregnant patient with Rh-negative blood may indicate the presence of antibodies to the Rh factor from an earlier transfusion with incompatible blood or from a previous pregnancy with an Rh-positive fetus.

A positive result indicates that the fetus may develop hemolytic disease of the newborn. As a result, repeated testing throughout the pregnancy is necessary to evaluate progressive development of circulating antibody levels.

Interfering factors

• Previous administration of dextran or I.V. contrast media (causing aggregation resembling agglutination)
• Hemolysis due to rough handling of the sample
• Blood transfusion or pregnancy within the past 3 months (possible presence of antibodies)