Direct Antiglobulin Test

The direct antiglobulin test (or direct Coombs' test) detects immunoglobulins (antibodies) on the surface of red blood cells (RBCs). These immunoglobulins coat RBCs when they become sensitized to an antigen, such as the Rh factor.

In this test, antiglobulin (Coombs') serum added to saline-washed RBCs results in agglutination if immunoglobulins or complement is present. This test is "direct" because it requires only one step- the addition of Coombs' serum to washed cells.

Purpose

• To diagnose hemolytic disease of the newborn (HDN)
• To investigate hemolytic transfusion reactions
• To aid differential diagnosis of hemolytic anemias, which can result from an autoimmune reaction or drugs or be congenital

Patient preparation

• If the patient is a neonate, explain to the parents that this test helps diagnose HDN.
• If the patient is suspected of having hemolytic anemia, explain that the test determines whether the condition results from an abnormality in the body's immune system, the use of certain drugs, or some unknown cause.
• Inform the adult patient that he need not fast before the test.
• Tell the patient (or a neonate's parents) that the test requires a blood sample and who will collect the blood or perform the venipuncture and when.
• Provide assurance that although the procedure may cause transient discomfort, collecting the sample takes only a few minutes.
• Withhold medications that may interfere with test results, including quinidine, methyldopa, cephalosporins, sulfonamides, chlorpromazine, diphenylhydantoin, ethosuximide, hydralazine, levodopa, mefenamic acid, melphalan, penicillin, procainamide, rifampin, streptomycin, tetracyclines, and isoniazid.

Procedure and posttest care

• For an adult, perform a venipuncture, and collect the sample in two 5-ml lavender-top tubes.
• For a neonate, draw 5 ml of cord blood into a red-top or lavender-top tube after the cord is clamped and cut.
• If a hematoma develops at the venipuncture site, apply warm soaks.
• Resume administration of medications withheld before the test.
• Tell the patient or the parents of the neonate with HDN that further tests will be necessary to monitor anemia.

Precautions

• Handle the sample gently to prevent hemolysis.
• Label the sample with the patient's full name, the hospital or blood bank number, the date, and the phlebotomist's initials.
• Send the ,sample to the laboratory immediately.

Normal findings

A negative test, in which neither antibodies nor complement appears on the RBCs, is normal.

Abnormal findings

A positive test on umbilical cord blood indicates that maternal antibodies have crossed the placenta and coated fetal RBCs, causing HDN. Transfusion of compatible blood lacking the antigens to these maternal antibodies may be necessary to prevent anemia.

In other patients, a positive test result may indicate hemolytic anemia and help differentiate between autoimmune and secondary hemolytic anemia, which can be drug-induced or associated with an underlying disease. A positive test can also indicate sepsis.

A weakly positive test may suggest a transfusion reaction in which the patient's antibodies react with transfused RBCs containing the corresponding antigen.

Interfering factors

• Hemolysis due to rough handling of the sample
• Quinidine, methyldopa, cephalosporins, sulfonamides, chlorpromazine, diphenylhydantoin, ethosuximide, hydralazine, levodopa, mefenamic acid, melphalan, penicillin, procainamide, rifampin, streptomycin, tetracyclines, and isoniazid (positive test results, possibly due to immune hemolysis).