Indirect Coombs Test
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The Indirect Coombs Test detects free-floating antibodies in the patient's serum that may target red blood cells (RBCs) — especially non-self (foreign) RBCs
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🧪 What is the Indirect Coombs Test?
The Indirect Coombs Test detects free-floating antibodies in the patient's serum that may target red blood cells (RBCs) — especially non-self (foreign) RBCs.
- It is used primarily to screen for antibodies prior to blood transfusion or pregnancy, ensuring compatibility.
- Unlike the Direct Coombs Test, which detects antibodies attached to RBCs, the Indirect test detects unbound antibodies in the serum.
❓ Why is the Indirect Coombs Test Done?
To:
- Screen for antibodies in the recipient before a blood transfusion (crossmatching)
- Evaluate Rh incompatibility during pregnancy (especially in Rh-negative mothers)
- Detect alloantibodies formed from previous transfusions or pregnancies
- Assess risk of hemolytic disease of the newborn (HDN)
🔬 How It Works
- Patient’s serum is mixed with RBCs of known antigen type.
- If antibodies are present, they will bind to the RBCs.
- Anti-human globulin (Coombs reagent) is added — causing agglutination if antibodies are present.
- Agglutination = positive test.
📊 Normal Results
Result | Interpretation |
|---|---|
Negative | ✅ Normal – No unexpected antibodies found; blood is compatible for transfusion |
Positive | ⚠️ Indicates the presence of antibodies against RBC antigens – may complicate transfusion or pregnancy |
📈 Interpretation of a Positive Test
Clinical Context | Meaning |
|---|---|
Pre-transfusion screening | Risk of transfusion reaction due to alloantibodies |
Pregnancy (Rh-negative mother) | Possible sensitization to Rh-positive fetal blood – risk of hemolytic disease of newborn |
Post-transfusion reaction | Development of new antibodies post-transfusion |
🧠 Associated Conditions
System | Related Conditions |
|---|---|
Hematologic | Alloimmune hemolytic anemia, autoimmune hemolytic anemia (indirect helps detect pre-formed antibodies) |
Obstetric | Hemolytic disease of the newborn (HDN), Rh incompatibility |
Transfusion Medicine | Delayed hemolytic transfusion reactions, cross-match incompatibility |
🔄 Related / Follow-Up Tests
- Direct Coombs Test (DAT) – Detects antibodies already bound to RBCs
- Rh Typing and ABO group
- Antibody identification panel – Identifies specific alloantibodies
- Crossmatch test – Checks compatibility between donor and recipient
- Bilirubin and CBC in newborn – If HDN is suspected
✅ Fasting Required?
Test | Fasting Requirement |
|---|---|
Indirect Coombs Test | ❌ No fasting required |
📝 Summary Table
Parameter | Description |
|---|---|
What | Detects free antibodies in the serum that may target RBC antigens |
Why | Pre-transfusion screening, Rh incompatibility in pregnancy, antibody screening |
Normal Result | Negative – No harmful antibodies detected |
Positive Result | Indicates risk of transfusion reaction or HDN |
Follow-up Tests | DAT, antibody ID panel, crossmatching |
Associated Conditions | Hemolytic disease of newborn, transfusion incompatibility |
Fasting Required | ❌ No |
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