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Lab Test: Coombs Test, Direct

    Lab Test
    • Coombs Test (Direct)
    • Detection of immunoglobulin and/or complement bound to red blood cells for the evaluation of antibody-induced hemolysis
    Reference Range
    • Adults and children:  Negative
    Indications & Uses
    • Hemolytic uremic syndrome (HUS):
      • Patients with HUS have Coombs' negative microangiopathic hemolytic anemia
    • Suspected autoimmune hemolytic anemia:
      • Positive direct Coombs test is a characteristic finding in autoimmune hemolytic anemia. 
      • The degree of the agglutination is proportional to the amount of antibody or complement on the RBCs and thus correlates with the probability of hemolysis. 
    • Suspected blood group antibody-antigen mismatch blood transfusion:
      • Positive direct Coombs test is diagnostic of immune hemolytic anemias, the most severe type being acute intravascular hemolysis after ABO incompatible RBC transfusion. 
    • Suspected cold autoimmune hemolytic anemia:
      • positive test indicates the presence of IgM autoantibodies that temporarily bind to RBCs, activate complement, and lead to the deposition of complement factor C3 on the cell surface. 
    • Suspected hemolytic disease of the newborn due to ABO immunization:
      • Occurs most commonly when the mother is group O and the baby is group A or B. 
    • Suspected paroxysmal cold hemoglobinuria:
      • Positive test indicates complement coating of anti-P antibodies (i.e., Donath-Landsteiner antibodies)
    • Mycoplasmal infection
    • Infectious mononucleosis
    • Hemolytic anemia after heart bypass
    Clinical Application
    • Most of the antibodies to RBCs are directed against the AB)/Rh blood grouping antigens, such as those that occur in hemolytic anemia of the newborn or blood transfusion of incompatible blood.  When a transfusion reaction occurs, the Coombs test can detect the patient's antibodies or complement components coating the transfused RBCs. 
    • The direct Coombs test demonstrates that RBCs have been attacked by antibodies in the patient's bloodstream.  The RBCs of patients suspected of having antibodies against RBCs are washed to eliminate any excess free gamma globulins.  Coombs serum is added to the RBCs.  If the RBCs have antibodies on them, Coombs serum will cause agglutination.  The test is read as positive with clumping on a scale of micro-positive to 4+. Results increased in hypergammaglobulinemia.
    • A direct Coombs test is important in establishing immune hemolysis, but for differential diagnosis an extended work-up is required which includes: 
      • Elution, autoabsorptions, serum studies to detect alloantibodies, and the utilization of drugs and/or drug-treated reagent cells to test serum/eluate.
    Related Tests
    • Coombs test, indirect - used to detect antibodies against RBCs in the serum.
    Drug-Lab Interactions
    • Antiphospholipid antibodies can cause a false-positive DAT.
    • Drugs that may cause false positive results include:  ampicillin, captopril, cephalosporins, chlorpromazine (Thorazine), chlorpropamide, hydralazine, indomethacin (Indocin), insulin, isoniazid (INH), levodopa, methyldopa (Aldomet), penicillin, phenytoin (Dilantin), procainamide, quinidine, quinine, rifampin, streptomycin, sulfonamides, and tetracyclines. 
    • Ticarcillin - a false positive Coombs' test
    • Clavulanic acid - a false positive Coombs' test
    Test Tube Needed
    • Red or lavender top tube
    • Collect venous blood specimen in an EDTA, lavender top tube and separate red cells to prevent uptake of complement components. 
    • Avoid clotted blood if possible.  If clotted blood is used, store specimen at 37°C until cells separate. 
    • Use venous blood from the umbilical cord to detect the presence of antibodies in the newborn. 
    • Apply pressure or a pressure dressing to the venipuncture site and assess the site for bleeding.
    Storage and Handling
    • May store specimen at 4°C for 1 week.
    What To Tell Patient Before & After
    • Explain the procedure to the patient.
    • Tell the patient that no fasting is required.
    • LaGow B et al., eds. PDR Lab Advisor. A Comprehensive Point-of-Care Guide for Over 600 Lab Tests.  First ed. Montvale, NJ: Thomson PDR; 2007.
    • Pagana K, Pagana TJ eds. Mosby's Manual of Diagnostic and Laboratory Tests. 5th Ed.  St. Louis, Missouri. 2014.

MESH Terms & Keywords

  • Coombs Test, Direct