When a DAT with polyspecific AHG is positive but a subsequent test shows no reactivity with anti-IgG or anti-C3D, what is the most likely explanation?

• A. Antibody titer dropped.
• B. Bacteria contaminated the saline.
• C. Complement can deteriorate overtime; use fresh patient plasma.
• D. Reagent expired.

Answer: (C)

Direct antiglobulin testing relies on detecting what is attached to red cells, either IgG or complement components like C3d. If an initial DAT using a polyspecific AHG is positive and a later test using anti-IgG and anti-C3d is negative for both, the most likely reason is that the complement that had been deposited on the cells has degraded over time in the stored sample. Complement is relatively labile, so with aging or improper storage the C3d signal can disappear, yielding negative results even though the first test was positive. Use fresh patient plasma or run the test promptly to ensure the complement component is intact.