For a patient with cold agglutinin disease, which component is likely detected in a repeat direct antiglobulin test?

In cold agglutinin disease, the primary antibody involved is IgM, which causes red blood cells to agglutinate at lower temperatures. This means that when performing a direct antiglobulin test (DAT) on a patient with this condition, the most likely component detected is C3d.

C3d is a complement component that can attach to red blood cells when they are coated in complement due to the action of IgM. In cold agglutinin disease, the IgM antibodies bind to the red blood cells and activate the complement pathway, leading to the deposition of complement components such as C3 on the surface of the red blood cells. When these cells are subject to a DAT, the presence of C3d indicates that complement was activated and suggests the immune-mediated process characteristic of cold agglutinin disease.

While IgM is indeed produced in this condition, it is typically not the component detected in the DAT because the test is focused on antibodies that are firmly bound to the red cells. Since IgM is a larger antibody and may not remain bound during washing steps of the DAT, it is less likely to show up in the final assessment compared to the complement portions.