Direct Coombs’ Test

Test Description

In some types of diseases, such as infectious mononucleosis and systemic lupus
erythematosus, and in sensitizations such as to the Rh factor, the red blood cells
become coated with antibodies. The direct Coombs’ test serves as a screening test
to determine whether such antibodies are attached to the patient’s red blood cells.
In this test a sample of the patient’s blood is mixed with Coombs’ antihuman
globulin serum. This serum is actually a rabbit serum which contains antibodies
against human globulins. When the patient’s blood is mixed with the rabbit serum,
clumping or agglutination occurs if antibodies are present on the patient’s red blood
cells. A common cause of a positive direct Coombs’ test is autoimmune hemolytic
anemia in which the person has antibodies against his own red blood cells.
The test has multiple purposes. It is used to screen blood during type and crossmatch
procedures. It can also be used to detect red blood cell sensitization to drugs
or blood transfusions, as in the testing for the occurrence of a hemolytic transfusion
reaction. In cases of suspected erythroblastosis fetalis, the test can be used to
determine the presence of antibodies to the newborn’s red blood cells.
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Normal Values

Negative
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Possible Meanings of Abnormal Values

Positive                                                        Negative

Elderly                                                          Hemolytic anemia
Erythroblastosis fetalis                             (nonautoimmune, non-drug-induced)
Hemolytic anemia (autoimmune,                Normal finding
drug-induced)
Infectious mononucleosis
Lymphomas
Neoplasms
Renal disorders
Rheumatoid arthritis
Systemic lupus erythematosus
Transfusion reaction
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Contributing Factors to Abnormal Values

• Hemolysis of the blood sample may alter test results.
• Drugs which may cause a positive direct Coombs’ test: ampicillin, captopril,
cephalosporins, chlorpromazine, chlorpropamide, ethosuximide, hydralazine,
indomethacin, insulin, isoniazid, levodopa, mefenamic acid, melphalan, methyldopa,
para-aminosalicylic acid, penicillin, phenylbutazone, phenytoin, procainamide,
quinidine, quinine sulfate, rifampin, streptomycin, sulfonamides, tetracyclines.
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Interventions/Implications

Pretest

• Explain to the patient the purpose of the test and the need for a blood sample to be drawn.
• No fasting is required before the test.

Procedure

• A 7-mL blood sample is drawn in a lavender-top (EDTA) collection tube.
• For newborns, a 5-mL umbilical cord blood sample is sufficient.
• Gloves are worn throughout the procedure.

Posttest

• Apply pressure at venipuncture site. Apply dressing, periodically assessing for continued
bleeding.
• Label the specimen and transport it to the laboratory.

• Report abnormal findings to the primary care provider.