Indirect Coombs Test (Antibody Screen)

Function

The Indirect Coombs Test, also known as the Indirect Antiglobulin Test (IAT), is primarily used to detect circulating antibodies against red blood cells (RBCs) in the recipient's or mother's serum. Unlike the Direct Coombs test, which looks for antibodies already attached to RBCs, this test identifies free-floating antibodies that could potentially attack foreign red cells. It is a cornerstone of pre-transfusion compatibility testing and prenatal screening.

Why it is Ordered

1. Prenatal Screening: It is routinely performed on pregnant women to check for Rh-incompatibility. If an Rh-negative mother carries an Rh-positive fetus, her body may produce antibodies (isoimmunization) that could cross the placenta and attack the baby's blood cells, leading to Hemolytic Disease of the Newborn (HDN).
2. Blood Transfusion: Before a patient receives a blood transfusion, a 'cross-match' is performed using the indirect Coombs method to ensure the recipient does not have antibodies that will react against the donor's blood.
3. Autoimmune Diagnosis: It helps in identifying certain types of autoimmune hemolytic anemias where the body produces antibodies against its own blood cells.

Associated Conditions

• Hemolytic Disease of the Newborn (HDN): Occurs when maternal antibodies destroy fetal RBCs.
• Hemolytic Transfusion Reaction: A life-threatening reaction following an incompatible blood transfusion.
• Systemic Lupus Erythematosus (SLE): An autoimmune condition that can lead to positive antibody screens.
• Chronic Lymphocytic Leukemia (CLL): Certain blood cancers can trigger the production of autoantibodies.