Blood Test Guide

Coombs Test (Direct & Indirect)

Name: Coombs Test (Direct & Indirect): What It Tests
Creator: Suman Konda
Published: 2026-07-05

The Coombs test detects antibodies attacking red blood cells. It is the key test for haemolytic anaemia and blood compatibility before transfusion.

Written and clinically reviewed by Suman Konda, PharmD, Clinical Pharmacist · Based on peer-reviewed sources · Editorial policy · Not medical advice

Last reviewed and updated: 01 August 2026 · How we check our content

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?

Frequently Asked Questions

What does the Coombs test detect?

It detects antibodies attached to red blood cells. The direct Coombs test checks for antibodies already on your cells (as in autoimmune haemolytic anaemia), while the indirect test screens blood before transfusion or in pregnancy.

Why is the Coombs test done in pregnancy?

In rhesus-negative mothers, the indirect Coombs test screens for antibodies that could attack a rhesus-positive baby's blood cells, guiding anti-D treatment to prevent haemolytic disease of the newborn.

What does a positive direct Coombs test mean?

It indicates the immune system is attacking your own red blood cells, which can occur in autoimmune haemolytic anaemia, certain medications, or some infections: requiring further evaluation.

What the two versions actually detect

The names are confusing and the distinction is worth getting straight. The direct antiglobulin test looks for antibody already stuck to the surface of the patient's own red cells, answering "are these cells coated?" The indirect test looks for free antibody circulating in plasma that could attack cells it has not yet met, answering "does this plasma contain antibodies against donor or fetal cells?" The direct test is used when investigating haemolysis in a patient; the indirect test is used for crossmatching before transfusion and for antibody screening in pregnancy.

A positive direct test is not automatically disease

A weakly positive direct antiglobulin test occurs in a small percentage of healthy blood donors and a larger proportion of hospital inpatients without any evidence of haemolysis. The test must therefore be read alongside markers of red cell destruction: haemoglobin, reticulocyte count, lactate dehydrogenase, bilirubin and haptoglobin. Haemolysis is diagnosed by that combination, with the antiglobulin test establishing whether the mechanism is immune. A positive test with entirely normal haemolysis markers usually needs no action.

Causes of a positive direct test

Autoimmune haemolytic anaemia, warm or cold type, which behave and are treated differently.
Drug-induced haemolysis, classically with penicillins, cephalosporins and some antivirals.
Underlying lymphoproliferative disease or lupus, where haemolysis can be the presenting feature.
Haemolytic disease of the newborn, and delayed transfusion reactions.
Recent intravenous immunoglobulin or anti-D, which can produce a positive result without significant haemolysis.

Why it matters in pregnancy

The indirect test underpins antenatal antibody screening. A pregnant woman who is RhD negative can form antibodies against a RhD positive fetus; these cross the placenta and destroy fetal red cells in a subsequent pregnancy. Routine screening at booking and again later identifies antibodies early, and anti-D prophylaxis given at the right times prevents sensitisation in the first place. It is one of the clearer examples of a laboratory test whose routine use has changed outcomes substantially.

References

The clinical information on this page is drawn from peer-reviewed sources indexed by the US National Library of Medicine. Links go to the source so you can read it yourself.

Coombs Test. In: StatPearls. Treasure Island (FL): StatPearls Publishing. NCBI Bookshelf NBK547707
Laboratory Evaluation of Immune Hemolytic Anemias. In: StatPearls. Treasure Island (FL): StatPearls Publishing. NCBI Bookshelf NBK606096