Which assay specifically measures the functional activity of the alternative complement pathway?

Which assay specifically measures the functional activity of the alternative complement pathway?

• A. AH50 assay
• B. CH50 assay
• C. Radial immunodiffusion
• D. Lytic assay with liposome

Answer: (A)

Assessing the functional activity of the alternative complement pathway requires a test that isolates the alternative route and measures actual lysis. The AH50 assay does this by evaluating how well a patient’s serum can drive hemolysis of erythrocytes through the alternative pathway, using conditions (Mg2+ with EGTA) that block the calcium-dependent classical pathway. The readout is the serum dilution that causes 50% lysis, providing a direct measure of the pathway’s functional activity. This is distinct from CH50, which tests the classical pathway under calcium-rich conditions using antibody-coated cells; radial immunodiffusion measures how much of a protein is present rather than whether it’s functioning; and lytic assays with liposomes can indicate lytic potential but are not the standard, pathway-specific test for the alternative route.

Assessing the functional activity of the alternative complement pathway requires a test that isolates the alternative route and measures actual lysis. The AH50 assay does this by evaluating how well a patient’s serum can drive hemolysis of erythrocytes through the alternative pathway, using conditions (Mg2+ with EGTA) that block the calcium-dependent classical pathway. The readout is the serum dilution that causes 50% lysis, providing a direct measure of the pathway’s functional activity. This is distinct from CH50, which tests the classical pathway under calcium-rich conditions using antibody-coated cells; radial immunodiffusion measures how much of a protein is present rather than whether it’s functioning; and lytic assays with liposomes can indicate lytic potential but are not the standard, pathway-specific test for the alternative route.