A decreased CH50 level with a normal AH50 level indicates a deficiency in which components?

• A. Decrease in components in the lectin pathway only
• B. Decrease in components in the alternative pathway only
• C. Decrease in components of the classical pathway only
• D. Decrease in components of both classical and alternative pathways

Answer: (C)

The key idea is that CH50 tests the classical (including the C1-based) pathway, while AH50 tests the alternative pathway. If the CH50 is reduced but the AH50 is normal, the defect lies in components used by the classical pathway before the convergence at C3, such as the C1 complex, C4, or C2. The alternative pathway components are intact, so that pathway functions normally. Therefore, the deficiency is in classical pathway components only.