CAMLPR Evaluation And Interpretation Competency Practice Test

This question hinges on how the direct antiglobulin test (DAT) appears in cold agglutinin disease, where the attack on red cells is driven primarily by IgM antibodies that fix complement. In cold agglutinin disease, IgM binds red cells at cold temperatures and activates the classical complement pathway, leaving complement fragments, especially C3 (C3b/C3d), attached to the red cell surface. The DAT uses reagents that detect either IgG or C3 on the surface of red cells. Because IgM is large and its binding is temperature-dependent, it is not reliably detected in standard DAT procedures, and it often dissociates at the warmer temperatures used in testing. In contrast, the C3 fragments that were deposited on the red cells remain and are detected by the DAT. As a result, the typical DAT pattern is positive for C3d (complement) and negative for IgG. So, the most consistent and characteristic finding is a DAT positive for C3d only, reflecting complement deposition rather than IgG coating.