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Introduction ⬇

The indirect Coombs test detects anti-RBC antibodies in the serum, whereas the direct Coombs test detects antigen–antibody complexes on the RBC membrane. Antibody identification is performed when the antibody screen or direct antiglobulin tests produce positive results and unexpected blood group antibodies need to be classified. Antibody identification tests are an important part of pretransfusion testing so that the appropriate antigen-negative blood can be transfused. These tests are also helpful for diagnosing the following conditions:

  1. Hemolytic disease of the newborn in which the RBCs of the infant are sensitized and exhibit antigen–antibody complexes in vivo

  2. Acquired hemolytic anemia in which an antibody is produced that coats the patient’s own cells (autosensitization in vivo)

  3. Transfusion reaction in which the patient may have received incompatible blood, which in turn has sensitized the donor’s and possibly the patient’s own RBCs

  4. RBC sensitization caused by drugs

The indirect Coombs test detects serum antibodies, reveals maternal anti-Rh antibodies during pregnancy, and can detect incompatibilities not found by other methods.

Normal Findings ⬆ ⬇

Procedure ⬆ ⬇

  1. Draw a 7-mL venous blood sample anticoagulated with EDTA and a 20-mL venous blood sample that is allowed to clot. Observe standard precautions.

  2. Label the specimen with the patient’s name, date, and test(s) ordered and place in a biohazard bag for transport to the laboratory.

  3. Notify the laboratory of diagnosis, history of recent and past transfusions, pregnancy, and any drug therapy, as indicated.

Clinical Implications ⬆ ⬇

  1. The direct Coombs test is positive (1+ to 4+) in the presence of the following conditions:

    1. Transfusion reactions

    2. Autoimmune hemolytic anemia (most cases)

    3. Cephalothin therapy (75% of cases)

    4. Drugs such as alpha-methyldopa (Aldomet), penicillin, insulin

    5. Hemolytic disease of newborn

    6. Paroxysmal cold hemoglobinuria

    7. In the presence of specific antibodies, usually from a previous transfusion or pregnancy, or nonspecific antibodies, as in cold agglutinants

  2. The indirect Coombs test is positive in the following conditions:

    1. Incompatible blood match

    2. Autoimmune or drug-induced hemolytic anemia

    3. Erythroblastosis fetalis hemolytic disease

Interventions ⬆ ⬇

Pretest Patient Care

  1. Explain purpose and procedure of test.

  2. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Posttest Patient Care

  1. Review test results; report and record findings. Modify the nursing care plan as needed. Counsel the patient regarding abnormal findings; explain the need for possible follow-up testing and treatment. Hemolytic disease of the newborn can occur when the mother is Rh negative and the fetus is Rh positive. Diagnosis is derived from the following information: mother is Rh negative, newborn is Rh positive, and the direct Coombs test is positive. Newborn jaundice results from Rh incompatibility, but more often, the jaundice results from an ABO incompatibility.

  2. Follow guidelines in Chapter 1 for safe, effective, informed posttest care.

Interfering Factors ⬆

A number of drugs may cause the direct Coombs test to be positive, including procainamide, quinidine, and methyldopa.