A DU test, also known as a Du phenotype test, is a blood test that is used to determine the presence of a weak form of the D antigen on red blood cells. The D antigen is a protein that is found on the surface of red blood cells. People who have the D antigen are Rh positive, while people who do not have the D antigen are Rh negative. The DU test is performed by adding an anti-D antibody to a blood sample. If the blood cells agglutinate, or clump together, then the person has the D antigen. If the blood cells do not agglutinate, then the person has a weak form of the D antigen, or Du phenotype. The DU test is usually performed in conjunction with a standard blood typing test. It is important to know whether a person has the Du phenotype because it can be difficult to distinguish between Rh negative and Du phenotype blood cells. This can be a problem in pregnancy, as it can lead to hemolytic disease of the newborn (HDN). HDN is a condition in which the mother's body produces antibodies against the baby's blood cells. This can happen if the mother is Rh negative and the baby is Rh positive. The antibodies can cross the placenta and attack the baby's red blood cells, causing them to break down. This can lead to anemia, jaundice, and even death in the baby. The DU test is usually performed in the following situations: During pregnancy, to determine the mother's blood type and Rh factor. Before a blood transfusion, to ensure that the donor blood is compatible with the recipient's blood type. In people with a family history of HDN. The DU test is a simple and painless procedure that is usually performed in a doctor's office or a laboratory. The patient will need to have a blood sample drawn from a vein in their arm. The results of the test are usually available within a few days. The DU test is a safe procedure. However, there are some risks associated with any blood test, including: Bleeding at the needle site. Infection. Allergic reaction to the needle or the test solution.