Expansion
Direct detects antibody on the cells, indirect detects antibody in the plasma
Expansion
- Direct antiglobulin test (direct Coombs): detects antibody or complement already bound to the patient’s red cells. It answers “are these cells coated”
- Indirect antiglobulin test: detects free antibody in the plasma, by incubating it with reagent red cells. Used for crossmatching and antenatal antibody screening
Causes of a positive direct test
- Autoimmune haemolytic anaemia
- Warm (IgG, extravascular, spleen): idiopathic, lymphoproliferative disease, lupus, drugs (methyldopa, penicillins), and it is the commonest type
- Cold (IgM and complement, intravascular and hepatic): Mycoplasma, Epstein-Barr virus, lymphoma, cold agglutinin disease
- Haemolytic disease of the newborn, from maternal alloantibody
- Haemolytic transfusion reaction, and delayed reactions days later
- Drug induced, by several mechanisms
- Passenger lymphocyte syndrome after transplantation, and intravenous immunoglobulin
- Healthy people: a weakly positive test occurs in around 1 in 10,000 donors, and more often in the elderly and in inpatients, so it is not synonymous with haemolysis
Confirming haemolysis requires the supporting evidence, whatever the Coombs result: raised reticulocytes, raised LDH, raised unconjugated bilirubin, low haptoglobin, and a film showing spherocytes in warm disease or agglutination in cold disease.
Coombs negative haemolysis does occur, in hereditary spherocytosis, G6PD deficiency, mechanical destruction, paroxysmal nocturnal haemoglobinuria and microangiopathy, so a negative test never excludes haemolysis.