A positive direct Coombs test indicates the presence of antibodies bound to which cells?

• A. Platelets
• B. Red blood cells
• C. Plasma proteins
• D. White blood cells

Answer: (B)

A positive direct Coombs test means immunoglobulin (or complement) is attached to the surface of red blood cells. When the anti-human globulin reagent is added, it binds those surface antibodies and cross-links the coated RBCs, causing agglutination. This indicates that red blood cells are coated with antibodies, which is a hallmark of immune-mediated hemolysis such as autoimmune hemolytic anemia or transfusion reactions. The test specifically detects antibodies on red blood cells, not on platelets, plasma proteins, or white blood cells.