Coombs Test (Anti-Globulin Test)

Types of Coombs Test

1. Direct Coombs Test (Direct Anti-Globulin Test)
   • Detects antibodies or complement on the surface of red blood cells (RBCs).
2. Indirect Coombs Test (Indirect Anti-Globulin Test)
   • Detects antibodies in the serum (unbound, floating in plasma).
   • Cannot detect complement because complement activates only on cell surfaces, not in plasma.

Test Procedures

Direct Coombs Test

• Steps:
  1. Add RBCs from the patient to the Coombs reagent.
  • Coombs Reagent: Contains antibodies to human globulin (could be antibodies to IgM, IgG, or complement).
  • Positive Test: Visual agglutination.

Indirect Coombs Test

• Steps: Two-stage process
  1. Add patient's plasma to RBCs with known antigens.
  2. Add Coombs reagent to the antigen-antibody complex.
  • Positive Test: Visual agglutination.

Uses

Direct Coombs Test

• Diagnoses conditions involving antibodies or complement on RBC surfaces such as:
  • Autoimmune Hemolytic Anemia
    • Warm Autoimmune Hemolysis (IgG on RBC surface)
    • Cold Autoimmune Hemolysis (IgM on RBC surface)
  • Drug-Induced Hemolysis
  • Alloimmune Hemolysis
    • Hemolytic Disease of the Newborn
    • Alloimmune Hemolytic Transfusion Reaction

Indirect Coombs Test

• Utilized in pre-transfusion testing:
  • Blood Typing
  • Cross-Matching
• Prenatal Antibody Screening to detect IgG antibodies that can cross the placenta and cause hemolytic disease of the newborn.

Summary

• Direct Coombs Test: Detects antibodies or complement on the RBC surface.
• Indirect Coombs Test: Detects antibodies in serum.

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