Coombs Test
On this page
Direct answer
Bridging invisible IgG coating into visible clumps is the single job of anti-human globulin (AHG) serum, the Coombs reagent. The direct antiglobulin test (DAT) adds AHG to the patient's own washed red cells and is positive when antibodies or complement sit on them in vivo — the defining test for haemolytic disease of the newborn and autoimmune haemolytic anaemia. The indirect antiglobulin test (IAT) incubates the patient's serum with reagent screening cells first, then adds AHG, and so detects free antibodies in vitro — the basis of antenatal antibody screening, the AHG phase of crossmatching and weak D confirmation. Same reagent, one crucial difference: in the direct test the antibody is already on the cell; in the indirect test, you put it there.
What you must remember
- DAT: patient's washed red cells + AHG; positive in HDN (cord blood), warm and cold autoimmune haemolytic anaemia, drug-induced haemolysis (methyldopa, penicillin) and haemolytic transfusion reactions.
- IAT: patient serum + screening cells, 37-degree incubation, wash, then AHG; used for antenatal anti-D screening, antibody identification panels, AHG crossmatching and weak D (Du) confirmation.
- Washing before AHG is not optional: residual unbound globulin neutralises the reagent and produces false negatives — the commonest technical failure in the practical exam.
- Collect the DAT sample into EDTA, whose calcium chelation prevents artefactual complement deposition on cells during storage.
- Warm autoimmune haemolytic anaemia coats cells with IgG, often with complement; cold agglutinin disease coats mainly with complement C3d.
- A positive DAT is not automatically haemolysis — confirm with reticulocytes, bilirubin, haptoglobin and blood film before calling haemolytic anaemia.
- Rh immunoprophylaxis logic: anti-D immunoglobulin to an Rh-negative mother at about 28 weeks and within 72 hours of delivery prevents her from making the anti-D that a subsequent DAT would detect in the next baby.
A jaundiced newborn, worked through
A term baby at 30 hours of life shows jaundice to the knees, total bilirubin 16 mg/dL mostly unconjugated, haemoglobin 12 g/dL. The mother is O negative, the baby groups A positive, and she received no antenatal anti-D. The laboratory pathway opens with a DAT on the baby's (EDTA) blood: red cells washed in saline several times, AHG added, and agglutination appears — antibodies are already on the baby's cells in vivo. Whose antibodies? Maternal IgG anti-D (or anti-A, since mother is O and baby is A) crossed the placenta and coated the baby's A-positive, D-positive cells.
The two possibilities are then separated by the mother's antibody screen and the baby's clinical picture. Rh haemolytic disease — maternal anti-D against D-positive cells — tends to be earlier, deeper anaemia with a strongly positive DAT; ABO incompatibility (group O mother, group A or B baby) gives a weakly positive DAT, spherocytes on the smear, and usually milder, self-limited haemolysis. Management follows severity, not the label: hour-specific phototherapy thresholds, intensive phototherapy, and exchange transfusion if the bilirubin trajectory outruns treatment. The laboratory's contribution — the DAT and the maternal screen — is what converts a jaundiced newborn into a diagnosed one within the first hour.
Where students slip
The mnemonic works until nerves strike, so anchor it: Direct = antibodies on the patient's own cells (in vivo); Indirect = antibodies free in serum, attached only during the test (in vitro). The second slip is skipping wash cycles to finish faster — the unwashed residual plasma proteins drink up the AHG and the test reads falsely negative, which in a newborn workup means a missed haemolytic disease. Third, over-reading a positive DAT as proof of haemolysis: some patients carry coating antibody with perfectly normal survival of red cells; haemolysis needs its own evidence.
Frequently asked questions
What is the difference between the direct and indirect antiglobulin tests?
DAT detects antibody or complement already bound to the patient's red cells in vivo; IAT detects free antibody in serum after in vitro incubation with screening cells.
Which clinical conditions produce a positive DAT?
Haemolytic disease of the newborn, warm and cold autoimmune haemolytic anaemia, haemolytic transfusion reactions and drug-induced haemolysis.
Why must cells be washed before adding AHG?
Unbound plasma globulins would neutralise the anti-human globulin reagent, causing false-negative results.
Why is an EDTA sample preferred for the DAT?
EDTA chelates calcium and prevents complement from depositing on red cells artefactually after collection, avoiding false positives.
What is the weak D (Du) test?
An indirect antiglobulin test confirming weakly expressed D antigen; donors confirmed positive are labelled Rh positive, recipients are treated as Rh negative.
Which drugs classically cause a positive DAT with haemolysis?
Methyldopa (via true autoantibody) and high-dose penicillin and cephalosporins (via drug adsorption to the red cell membrane).