Final answer:
A positive DAT with Anti-IgG can lead to a false-positive result in a Weak D test because the Anti-IgG bound to the red blood cells may cause agglutination, interfering with accurate RhD antigen detection.
Step-by-step explanation:
If a patient has a positive direct antiglobulin test (DAT) with Anti-IgG, performing a Weak D test on the patient's cells can potentially yield a false-positive result. The DAT indicates the presence of anti-immunoglobulin antibodies bound to the red blood cells' surface in vivo. The Weak D test is used to detect a weak expression of the D antigen in the Rh blood group system. However, the presence of Anti-IgG coating the red blood cells in the DAT may cause agglutination in the Weak D test, leading to a false-positive result for RhD antigen presence.
The DAT is used to detect conditions such as autoimmune hemolytic anemia, hemolytic disease of the newborn, or reactions following a blood transfusion. In this test, anti-human globulin reagent is added to the patient's red blood cells, which will agglutinate if antibodies or complement are present on the cells' surface. On the other hand, the Weak D test identifies the weaker expression of the D antigen, which may not be detected by standard Rh typing methods, and it is important for correctly classifying RhD status and preventing Rh incompatibility in blood transfusions.
When a Weak D test is inadvertently performed on red blood cells already coated with Anti-IgG, it can give a false-positive result. This interference occurs because the test will detect the agglutination caused by the bound antibodies rather than the D antigen itself, making the test result unreliable for RhD antigen detection.