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Diagnostic Tests: Coombs’ Agglutinin Test
Overview — What This Test Is
The Coombs’ test, also known as the antiglobulin test, detects antibodies directed against red blood cells. It identifies whether RBCs are coated with immunoglobulin or complement. There are two forms:
- Direct Coombs’ test (DAT): Detects antibodies attached to the patient’s RBCs.
- Indirect Coombs’ test: Detects antibodies circulating in the patient’s serum.
The Direct Coombs’ test is the one used in evaluating hemolysis.
Clinical Significance
The Coombs’ test is central to diagnosing autoimmune hemolytic anemia (AIHA). A positive direct Coombs’ test indicates that red blood cells have been coated with IgG and/or complement, leading to premature destruction. When the patient’s RBCs agglutinate after addition of the Coombs reagent, it confirms immune-mediated hemolysis.
A positive Coombs’ test is also helpful in distinguishing autoimmune hemolysis from non-immune causes such as mechanical destruction or enzymatic defects.
Most Common Cause of a Positive Coombs’ Test
Between 30–50% of Coombs’ positive hemolytic anemia cases are idiopathic, meaning no clear underlying cause can be identified.
Among identifiable causes, the most common are drug-induced hemolysis, especially due to:
- Penicillin
- Quinidine
- Alpha-methyldopa
- Sulfa drugs
- Aspirin
These medications may either cause antibodies to form against RBCs or bind to RBCs themselves, triggering an immune response and hemolysis.
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