Direct vs indirect Coombs test
| Test | What it detects | When ordered |
|---|---|---|
| Direct Coombs (DAT) | Antibodies or complement already bound to the patient's red blood cells | Suspected haemolytic anaemia, haemolytic disease of the newborn, drug-induced haemolysis |
| Indirect Coombs (IAT) | Antibodies in the patient's serum that could bind to donor red blood cells | Pre-transfusion compatibility testing, antenatal screening |
What does a positive direct Coombs mean?
Causes of a positive DAT
A positive direct Coombs means antibodies or complement proteins are coating the patient's red blood cells. Causes include: autoimmune haemolytic anaemia (the immune system attacks its own red cells), drug-induced haemolysis (methyldopa, penicillin, quinidine), haemolytic disease of the fetus and newborn (Rh incompatibility), alloimmune haemolysis after blood transfusion. The red cells coated with antibodies are destroyed by the spleen, causing anaemia and jaundice.
Haemolytic anaemia blood test pattern
| Test | Finding in haemolysis |
|---|---|
| Haemoglobin | Low (anaemia) |
| Bilirubin (unconjugated) | Raised (from red cell breakdown) |
| LDH | Raised (released from destroyed red cells) |
| Haptoglobin | Low (consumed by free haemoglobin) |
| Reticulocyte count | Raised (bone marrow compensating) |
Questions to ask your doctor
- Is my positive Coombs test causing my anaemia?
- Is this autoimmune, drug-induced, or from a transfusion reaction?
- Do I need a haematology referral?
- Should I stop any medication that may be causing this?
Medical Disclaimer: For educational purposes only. Always consult a qualified healthcare professional for diagnosis and treatment.