The Indirect Coombs Test (also known as the Indirect Antiglobulin Test, IAT) is a laboratory test used to detect antibodies against red blood cells that are present in the bloodstream, but not yet attached to red blood cells. It is commonly used in blood transfusion compatibility testing and prenatal screening. The test involves mixing a patient’s serum with red blood cells of known antigenic type. If antibodies are present in the serum, they will bind to the red blood cells, and the addition of Coombs reagent will cause agglutination (clumping), indicating a positive result. This test is used to identify alloantibodies, which could lead to transfusion reactions or hemolytic disease of the newborn.