Which laboratory test is used to test for terminal complement deficiency?

• A. C3 level
• B. CH50 assay
• C. ANA test
• D. ESR

Answer: (B)

Testing terminal complement deficiency focuses on the ability of the MAC to form and lyse targets. The CH50 assay measures the functional activity of the classical pathway by seeing how well serum can lyse antibody-coated sheep red blood cells, which requires intact components C5–C9 to form the membrane attack complex. A low CH50 indicates a deficiency in the terminal components, explaining why the body struggles to form MAC and why individuals are susceptible to Neisseria infections. Upstream components like C3 can be normal in terminal deficiencies because they’re not directly affected, so a low C3 isn’t as specific. The ANA test and ESR are nonspecific and don’t assess the terminal complement pathway.