From the Guidelines
The Coombs test is a crucial diagnostic tool used to detect antibodies attached to red blood cells, with the direct Coombs test identifying antibodies already bound to red blood cells and the indirect Coombs test detecting free antibodies in the patient's serum. The direct Coombs test is useful for diagnosing conditions like autoimmune hemolytic anemia, hemolytic disease of the newborn, and transfusion reactions, as it identifies antibodies already bound to red blood cells in the body 1. In this test, a patient's red blood cells are mixed with Coombs reagent (anti-human globulin), and agglutination indicates a positive result. The indirect Coombs test, on the other hand, detects free antibodies in the patient's serum that can bind to red blood cells, and is commonly used for blood typing, crossmatching before transfusions, and prenatal screening 1. This test involves mixing the patient's serum with known red blood cells, incubating to allow antibody binding, washing away unbound antibodies, then adding Coombs reagent. Both tests are essential diagnostic tools in hematology and transfusion medicine, helping to identify immune-mediated hemolytic conditions and ensure safe blood transfusions, as emphasized in various clinical guidelines, including those for the management of anemia in cancer patients 1. Key points to consider when using the Coombs test include:
- The test's ability to detect antibodies attached to red blood cells, which is critical for diagnosing certain conditions
- The distinction between the direct and indirect Coombs tests, and their respective uses
- The importance of the Coombs test in ensuring safe blood transfusions and identifying immune-mediated hemolytic conditions.
From the Research
Direct Coomb's Test
- The Direct Antiglobulin Test (DAT), also known as the Direct Coomb's test, detects antibodies bound to red cells 2, 3.
- It is a valuable test aiding in the diagnosis of Hemolytic Disease of the Fetus and Newborn (HDFN) 2.
- The DAT is used to identify situations where there is an incompatibility between the blood types of the mother and the fetus, which can lead to the destruction of fetal red blood cells 2, 3.
- The test is performed on cord blood or newborn blood samples if the mother is Rhesus D negative or has non-A/B red cell alloantibodies 2.
Indirect Coomb's Test
- The Indirect Coomb's test, also known as the Indirect Antiglobulin Test (IAT), detects the presence of antibodies in the serum that are capable of binding to red blood cells 3, 4.
- It is used to identify antibodies against red blood cell antigens in the serum of pregnant women, which can help predict the risk of HDFN 3.
- The IAT is also used to screen for antibodies in blood donors and to investigate the cause of hemolytic anemia 3, 4.
Clinical Significance
- A positive DAT result can indicate the presence of HDFN, which can cause fetal hydrops during pregnancy or neonatal jaundice after birth 2.
- A negative DAT result does not rule out the possibility of HDFN, as some cases may have a negative test result due to low levels of antibody sensitization or other factors 4, 5.
- The DAT has a sensitivity of 38.5% and specificity of 98.5% for detecting significant hemolysis, and a positive predictive value of 58.8% for significant hemolysis 5.