The indirect Coombs test titre quantifies circulating IgG antibodies against red blood cells in a patient’s serum. The laboratory dilutes serum serially and identifies the highest dilution that still produces agglutination; that dilution is reported as the titre (for example 1:32). It helps detect and monitor alloimmunization in pregnancy and assess compatibility before blood transfusion.

An ICT titre is done by collecting blood and separating the serum, then making serial dilutions of the serum and incubating each with reagent red cells of known antigen. After washing, anti‑human globulin (Coombs reagent) is added; agglutination shows antibody binding. The highest dilution that still agglutinates is recorded as the antibody titre, used to monitor alloimmunization.

A Coombs titer is a blood test measuring antibodies that bind to red blood cells. It determines the highest serum dilution that still causes agglutination (a positive result). The Direct Coombs detects antibodies already on a patient’s red cells; the Indirect Coombs measures free antibodies in serum. It helps diagnose autoimmune hemolytic anemia, hemolytic disease of the newborn, and guides transfusion compatibility; higher titers mean more antibody.

A positive indirect Coombs (indirect antiglobulin) test means circulating anti–red blood cell antibodies are present in the serum. Common causes include alloimmunization after prior blood transfusion or pregnancy (maternal antibodies to fetal RBCs), risk of hemolytic disease of the newborn, recent transfusion reactions, certain drugs that induce antibody formation, and immune disorders producing RBC-directed antibodies.

A critical titre is the antibody concentration cutoff, measured by serial serum dilutions, that indicates protective immunity or warrants clinical action. It’s reported as the highest dilution that still yields a positive reaction (reciprocal value). Clinicians use critical titres to decide vaccination, boosters, or further evaluation—for example, determining immunity or need for treatment in infectious disease and transfusion contexts.

A positive indirect Coombs in pregnancy means maternal IgG antibodies against fetal red‑cell antigens (alloimmunization), risking hemolytic disease of the fetus/newborn. Management includes identifying antibody type and serial titers, fetal surveillance (middle cerebral artery Doppler for anemia), and interventions if needed (intrauterine transfusion, early delivery, neonatal phototherapy or exchange transfusion). Anti‑D prevents sensitization but won’t reverse it.