Which complement deficiency will have a normal AH50 and low CH50?

A complement deficiency that results in a normal AH50 (Alternative Hemolytic complement pathway) and a low CH50 (Classical Hemolytic complement pathway) indicates a specific defect in the classical pathway of complement activation. The CH50 test assesses the functionality of the classical pathway, which is primarily dependent on complement components C1, C2, C4, and C3. A low CH50 suggests that the functionality of the classical pathway is compromised, typically due to a deficiency in one of its components.

In the case of C4 deficiency, this is characterized by a low CH50 because C4 is essential for the classical pathway's ability to lyse red blood cells (used in the CH50 test). On the other hand, the AH50 test evaluates the alternative pathway, which is less affected by the deficiency of classical pathway components.

Thus, a patient with C4 deficiency would present with a normal AH50 because the alternative pathway can still function independently of the classical pathway components, while the CH50 remains low due to the lack of C4, which is crucial for the classical pathway’s function.

This distinctive pattern—normal AH50 paired with low CH50—is characteristic of C4 deficiency, confirming that the answer aligns correctly with