Which test is most commonly used to detect antibodies attached to a patient's red blood cells in vivo?

The direct antiglobulin test (DAT) is specifically designed to detect antibodies that are bound to a patient’s red blood cells in vivo. This test is crucial in situations such as hemolytic anemia, Rh incompatibility, and autoimmune hemolytic anemia, where the presence of these antibodies might lead to the destruction of red blood cells.

In performing the direct antiglobulin test, a blood sample is taken from the patient, and the red blood cells are washed to remove any unbound serum proteins. The washed cells are then incubated with antihuman globulin (also known as Coombs reagent). If antibodies or complement have attached to the red blood cells, the Coombs reagent will cause agglutination, indicating a positive result.

This test is distinct from the indirect antiglobulin test, which assesses antibodies present freely in the serum rather than those bound to the cells. Complement fixation is not specific to detecting antibodies on red blood cells and is generally used for identifying specific infections. Immunofluorescence is a more specialized technique used for various diagnostic purposes, including detecting specific antibodies, but is not primarily used for the direct assessment of antibodies bound to RBCs in vivo. Thus, the direct antiglobulin test is the most relevant and