Coombs Test Interpretation
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Direct answer
The Coombs (antiglobulin) test uses antihuman globulin to bridge otherwise invisible molecules: the direct antiglobulin test (DAT) detects antibodies or complement already coating the patient's red cells in vivo, while the indirect antiglobulin test (IAT) detects free antibody in serum by reacting it with reagent red cells. A positive DAT with IgG (± C3d) defines warm autoimmune haemolytic anaemia — spherocytes, splenic extravascular haemolysis; isolated C3d positivity defines cold antibody disease, whether IgM cold agglutinins (complement remains after IgM detaches) or the biphasic Donath-Landsteiner antibody of paroxysmal cold haemoglobinuria. The IAT anchors antenatal antibody screening, crossmatching and anti-D prophylaxis, making the two tests a fixed FMGE pair.
What you must remember
- DAT essentials: antihuman globulin added to washed patient red cells; polyspecific AHG screens for IgG and C3d, monospecific reagents then identify which is present.
- Warm AIHA pattern: IgG with or without C3d, spherocytes on the smear, splenic extraction, secondary to lymphoproliferative disease, autoimmune disorders or drugs; responds to steroids.
- Cold agglutinin pattern: C3d only (the IgM fixes complement at cold temperatures then detaches), acrocyanosis, red cell agglutination in the EDTA tube, secondary to mycoplasma pneumonia, infectious mononucleosis or lymphoma.
- Paroxysmal cold haemoglobinuria: biphasic IgG anti-P (Donath-Landsteiner) antibody, intravascular haemolysis with haemoglobinuria in children after viral infection; DAT positive with C3d alone.
- Neonatal haemolytic disease: a positive DAT on the newborn's cells confirms maternally derived antibody (anti-D or ABO); ABO disease is milder because IgG-coated cells clear slowly.
- IAT applications: antenatal antibody screening, antibody identification, pre-transfusion crossmatch and anti-D titre monitoring in sensitised pregnancies.
- Prevention of Rh alloimmunisation: anti-D immunoglobulin at 28 weeks of gestation and within 72 hours of delivery (or after any sensitising event) for Rh-negative mothers.
A jaundiced newborn, read through the tube
An O-positive mother delivers a B-positive baby who develops jaundice at 24 hours with a mild anaemia and spherocytes on smear. The baby's DAT is weakly positive: maternal IgG anti-B has crossed the placenta — ABO haemolytic disease, usually mild, managed with phototherapy and, occasionally, exchange transfusion. Contrast the Rh-sensitised scenario: an Rh-negative mother, previously sensitised, whose anti-D titres (measured by IAT during pregnancy) climbed; the baby presents earlier and sicker, with a strongly positive DAT, and the prevention that failed — anti-D not given after the last delivery or miscarriage — is the lesson. The same laboratory bridge explains both: the DAT reads what is on the baby's cells, the IAT read what was in the mother's serum months before.
Where students slip
The component question decides marks: warm AIHA shows IgG ± C3d, cold agglutinin disease shows C3d alone, and candidates who answer "IgM on the DAT" forget that IgM cannot be detected once it detaches at 37°C. DAT-negative AIHA exists (low-affinity antibodies, under 5 per cent of cases) and should temper certainty. Finally, the drug mechanisms are testable pairs: penicillin acts as a hapten on the cell (DAT IgG), methyldopa induces true autoantibody (DAT IgG), while cephalosporins fix complement non-specifically.
Frequently asked questions
What does a positive direct antiglobulin test mean?
Antibody or complement is coating the patient's red cells in vivo — autoimmune haemolysis, haemolytic disease of the newborn, a drug-induced mechanism or a transfusion reaction.
Which pattern characterises cold agglutinin disease?
C3d alone on the DAT, because IgM fixes complement at cooler temperatures then detaches, leaving complement as the detectable footprint.
When is the indirect antiglobulin test used?
To detect free antibody in serum — antenatal antibody screening, antibody identification, pre-transfusion crossmatching and anti-D titre monitoring.
What is the Donath-Landsteiner antibody?
A biphasic IgG anti-P antibody binding in the cold and lysing cells on warming, causing paroxysmal cold haemoglobinuria with a C3d-positive DAT.
When is anti-D immunoglobulin given?
At around 28 weeks of gestation and within 72 hours of delivery or any sensitising event, to prevent RhD alloimmunisation in Rh-negative mothers.