Fresh serum (less than 48 hours old) must be used for compatibility testing in order to preserve which component?

• A. immunoglobulins
• B. fibrinogen
• C. antibodies
• D. complement

Answer: (D)

Complement proteins are required for certain compatibility testing reactions, and they’re unusually delicate. Fresh serum keeps these components active, so the classical pathway can proceed if antibodies bind to donor red cells. Over time, complement activity declines or becomes inactivated, which can mask true compatibility results. Fibrinogen is not present in serum (it’s consumed during clotting), and antibodies are more stable but aren’t the factor that must be preserved for these tests. So preserving complement with freshly drawn serum ensures the test can accurately reveal antibody–antigen interactions that rely on complement-mediated effects.