Indirect Coomb's Test is a blood test that is used to determine the compatibility of the antibodies that are present in the blood of different people. Indirect Coomb's test is also known as indirect antiglobulin test or IAT and it looks at the plasma of the blood and detects certain antibodies present in the plasma. The test is conducted before a blood transfusion to ensure that donor's blood is compatible with the recipient's blood. The test is also done during the early stages of pregnancy to determine Rh factor of the mother's blood.