The indirect antiglobulin test (IAT, indirect Coombs', antibody screening test) is used primarily to screen blood samples for unexpected circulating antibodies that may be reactive against transfused red blood cells.

In this test, the client's serum serves as the source of antibody, and the red cells to be transfused serve as the antigen. The test is performed by incubating the serum and red cells in the laboratory (in vitro) to allow any antibodies that are present every opportunity to attach to the red cells. The cells are then washed with saline to remove any unattached serum globulins, and AHG is added. If the client's serum contains an antibody that reacts with and attaches to the donor red cells, the AHG will cause the antibody-coated cells to agglutinate.

If no agglutination occurs after addition of AHG, then no antigen-antibody reaction has occurred. The serum may contain an antibody, but the red cells against which it is tested do not have the relevant antigen. Thus, the reaction is negative.¹⁹

• Negative (no agglutination)

• Recent administration of dextran, whole blood or fractions, or intravenous contrast media may result in a false-positive reaction.
• Drugs that may cause false-positive reactions are cephalosporins, insulin, isoniazid, levodopa, mefenamic acid, methyldopa, methyldopa hydrochloride, penicillins, procainamide hydrochloride, quinidine, rifampin, sulfonamides, and tetracyclines.

• Antibody screening and cross-matching before blood transfusions, especially to detect antibodies whose presence may not be elicited by other methods such as ABO and Rh typing
• Determination of antibody titers in Rh-negative women sensitized by an Rh-positive fetus
• Testing for the weak Rh variant antigen D
• Detection of other antibodies in maternal blood that may be potentially harmful to the fetus

Nursing Care Before the Procedure

Client preparation is the same as that for any study involving the collection of a peripheral blood sample (see Appendix I).

• Exposure to substances that may cause false-positive reactions should be noted. The medication history should also be noted.

A venipuncture is performed and the sample collected in a red-topped tube or other blood collection tube, depending on laboratory preference. The sample must be handled gently to avoid hemolysis and sent promptly to the laboratory.

Nursing Care After the Procedure

Care and assessment after the procedure are the same as for any study involving the collection of a peripheral blood sample.

• Complications and precautions: Note and report a positive value for antibody detection, especially in a pregnant woman. Inform the client that further testing will be undertaken to identify the antibodies.