In Coombs test (antiglobulin test) interpretation, a positive direct Coombs test (DCT) with negative indirect Coombs test (ICT) indicates:
- A Free antibodies in serum against red cell antigens (pre-transfusion compatibility testing failure)
- B Incompatible crossmatch due to alloantibodies in recipient serum
- C Past transfusion with sensitisation to donor antigens without active haemolysis
- D Antibody or complement already bound to patient's own red cells in vivo ✓
Explanation
The direct Coombs test (DCT) detects antibodies or complement components already bound to the patient's red blood cells in vivo; a positive DCT (with anti-IgG or anti-C3d antiglobulin reagent causing agglutination) indicates in vivo sensitisation. This occurs in autoimmune haemolytic anaemia, haemolytic disease of the newborn (HDN), and drug-induced haemolysis. The indirect Coombs test (ICT) detects free antibodies in serum capable of coating donor RBCs in vitro (used for pre-transfusion crossmatching and antenatal antibody screening). Positive DCT with negative ICT means in vivo sensitisation with no or low-titre free circulating alloantibody.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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Written and medically reviewed by the StethoPrep medical team.