What this test measures
The Indirect Coombs Test (Indirect Antiglobulin Test, IAT) looks for free antibodies against red blood cell antigens circulating in your plasma. Your serum is incubated with screening cells of known antigen profile; if antibodies are present they bind the cells, and Coombs antiglobulin reagent then visualises the binding by agglutination.
The test is used in three main settings: (1) Pre-transfusion antibody screening — to find clinically significant antibodies (anti-Kell, anti-Duffy, anti-Kidd, irregular anti-D, etc.) that could cause haemolytic transfusion reactions if blood is mismatched; (2) Antenatal antibody screening in pregnancy — to detect maternal antibodies that could cross the placenta and cause haemolytic disease of the newborn; (3) Crossmatching donor blood for transfusion.
Why it matters
For Rh-negative pregnant Indian women, antenatal Indirect Coombs (antibody screen) at the booking visit and at 28 weeks identifies those who have already developed anti-D antibodies — typically from a prior un-prophylaxed pregnancy. Once anti-D is detected, anti-D immunoglobulin prophylaxis no longer helps (the immune system is already sensitised), and the pregnancy is at risk of haemolytic disease of the newborn. Foetal medicine referral and serial middle cerebral artery Doppler are then needed to monitor foetal anaemia.
For patients about to receive a transfusion, a positive Indirect Coombs (other than anti-D in known Rh-negative patients) means standard cross-match procedures need to be expanded — finding a fully compatible unit takes longer and the patient must wait. For patients with chronic transfusion needs (thalassemia major, sickle cell disease), regular IAT screening tracks alloantibody development over time.
How to prepare
No fasting required. Disclose any prior transfusions, prior pregnancies (especially if anti-D was missed), bone marrow transplant, and current medications. If you are Rh-negative and pregnant, mention this so the lab can specifically report anti-D status.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Indirect Antiglobulin Test[1][2] | Negative | — | Positive — antibody present in plasma. Further work-up identifies the specific antibody (anti-D, anti-K, anti-Fya, anti-Jka, etc.) and titre. Clinically significant antibodies require careful blood selection for transfusion and close foetal monitoring in pregnancy. |
| Anti-D titre (if positive in Rh-negative pregnancy)[1] | Critical titre typically ≥ 1:16 | — | Titre ≥ 1:16 — foetal medicine referral; serial MCA Doppler to detect foetal anaemia. Intrauterine transfusion may be required for severe foetal anaemia. |
Direct vs Indirect Coombs — which is which
| Feature | Direct Coombs (DAT) | Indirect Coombs (IAT) |
|---|---|---|
| Looks for | Antibodies already on red cells | Free antibodies in plasma |
| Used in | Diagnosing haemolysis (AIHA, HDN, drug) | Antibody screen, cross-match, antenatal screen |
| Sample tested | Patient's red cells | Patient's serum/plasma |
| Key result | Positive = haemolysis cause | Positive = sensitisation / alloimmunisation |
Frequently asked questions
Why am I being asked to do an Indirect Coombs?
Most commonly during pregnancy (to check for antibodies that could harm the baby) or before a transfusion or surgery (to ensure donor blood will be compatible).
Is the Indirect Coombs different from the Direct Coombs?
Yes. Direct Coombs checks whether antibodies are already attached to your red cells (used in haemolytic anaemia diagnosis). Indirect Coombs checks for free antibodies in your plasma (used in cross-matching and antenatal screening).
Do I need to fast?
No.
I am Rh-negative and pregnant — when should I do the test?
At the booking visit (6–12 weeks) and again at 28 weeks before anti-D prophylaxis. If you have had previous pregnancies, additional testing may be done in each trimester.
My antibody screen is positive — what does that mean?
Your plasma contains an antibody against a red cell antigen. The lab will identify the specific antibody and measure its titre. The implications depend on which antibody it is and the clinical context (pregnancy or transfusion).
Can a positive IAT affect my baby?
Yes — clinically significant antibodies (anti-D, anti-Kell, anti-c, etc.) can cross the placenta and cause foetal anaemia and haemolytic disease of the newborn. Your obstetrician will arrange foetal medicine review and serial monitoring.
I am about to have surgery — why is this needed?
Before any planned blood transfusion, the antibody screen helps the blood bank select compatible donor blood. A negative screen means standard cross-match takes minutes; a positive screen may take hours to find the right unit.
How long does the report take?
Most NABL labs report a routine antibody screen within 4–6 hours. Antibody identification (if positive) may take 24–48 hours.
Related Hematology / Anemia tests
Tests commonly ordered alongside INDIRECT COOMBS TEST, or that help interpret an unexpected result.
Sources & references
- NCBI StatPearls — Indirect Coombs Test · accessed 2026-05-30T00:00:00.000Z
- BSH — Pretransfusion Compatibility Procedures · accessed 2026-05-30T00:00:00.000Z
- RCOG Green-top Guideline — Red Cell Antibodies in Pregnancy · accessed 2026-05-30T00:00:00.000Z
- NIH MedlinePlus — Coombs Test · accessed 2026-05-30T00:00:00.000Z
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