The CH50 test measures which of the following?

• A. Functioning of the lectin pathway only
• B. Genetic deficiencies of any of the complement components
• C. Functioning of both the classical and alternative pathways
• D. Patient serum required to lyse 50% of sensitized sheep RBCs

Answer: (D)

The CH50 is a functional assay of the classical complement pathway. In this test, patient serum is incubated with antibody-sensitized sheep red blood cells; if the classical pathway is intact, the serum’s complement components will eventually lyse the target cells. The amount of patient serum required to achieve 50% lysis is the CH50 value. A lower CH50 means reduced classical pathway activity, which can result from deficiencies or inhibitors of classical components. This assay specifically assesses the classical pathway, not the lectin or alternative pathways (which require separate tests). While a deficient CH50 can reflect genetic deficiencies, the test itself measures functional activity, not genetic status. Therefore, the correct description is the patient serum required to lyse 50% of sensitized sheep RBCs.