27/09/2025
Coombs Test (Direct and Indirect Antiglobulin Test)
1. Objective:
The objective of the Coombs test was to detect the presence of antibodies or complement proteins bound to the surface of red blood cells (RBCs). The Direct Coombs Test (DAT) aimed to identify antibodies attached directly to the patient's RBCs, while the Indirect Coombs Test (IAT) was performed to detect free antibodies present in the patient's serum that could react with RBC antigens.
2. Principle:
The principle of the Coombs test was based on the detection of antigen–antibody reactions. In the Direct Coombs Test, RBCs coated with immunoglobulin (IgG) or complement were mixed with Coombs reagent (anti-human globulin). The reagent cross-linked the sensitized RBCs, causing visible agglutination.
In the Indirect Coombs Test, the patient's serum containing antibodies was first incubated with donor or test RBCs. If the antibodies in the serum bound to the antigens on the RBCs, the subsequent addition of Coombs reagent caused agglutination. The appearance of agglutination indicated a positive test result.
3. Materials:
• Patient's blood sample (for Direct Coombs Test)
• Patient's serum (for Indirect Coombs Test)
• Test RBCs (antigen-positive cells for IAT)
• Coombs reagent (anti-human globulin serum)
• Test tubes
• Pipettes
• Centrifuge
• Microscope
• Isotonic saline solution
• Glass slides
4. Procedure (Microscopic):
a) Direct Coombs Test (DAT):
1. A blood sample was collected from the patient, and RBCs were washed three times with isotonic saline to remove unbound antibodies.
2. The washed RBC pellet was suspended in saline to make a 2–5% RBC suspension.
3. Two drops of Coombs reagent were added to the washed RBC suspension in a test tube.
4. The tube was mixed gently and centrifuged for 1–2 minutes at 1000 rpm.
5. The sediment was examined microscopically for agglutination.
6. The presence of clumping indicated a positive Direct Coombs Test.
b) Indirect Coombs Test (IAT):
1. Two drops of the patient’s serum were mixed with one drop of test RBC suspension in a test tube.
2. The mixture was incubated at 37°C for 30 minutes to allow antibodies to bind to RBC antigens.
3. After incubation, the RBCs were washed three times with saline to remove unbound antibodies.
4. Two drops of Coombs reagent were added to the washed RBCs.
5. The tube was centrifuged for 1–2 minutes and then examined microscopically.
6. Agglutination of RBCs indicated a positive Indirect Coombs Test.
5. Result:
• Positive Result: Visible clumping or agglutination of RBCs was observed under the microscope, indicating the presence of antibodies bound to RBCs (DAT) or free antibodies in the serum (IAT).
• Negative Result: No agglutination was observed, indicating the absence of significant antibodies.
6. Uses:
• To diagnose autoimmune hemolytic anemia by detecting autoantibodies attached to the patient's RBCs.
• To screen for hemolytic disease of the newborn (HDN) caused by Rh incompatibility between mother and fetus.
• To cross-match blood during transfusion to prevent hemolytic transfusion reactions.
• To detect antibodies in patients receiving multiple transfusions.
• To monitor patients for immune-mediated drug reactions affecting RBCs.
7. Consultation:
If the test result was positive, the patient was advised to consult a hematologist or immunologist for further evaluation and management. In cases of suspected hemolytic disease of the newborn, immediate neonatal care was recommended. For blood transfusion-related incompatibilities, the blood bank physician was consulted to ensure safe transfusion practices. The patient was counseled about possible causes and preventive measures, including Rh immunoglobulin administration in Rh-negative mothers.
