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Complement, AP50 total alternative pathway & CH50/THC total classical pathway function

Category Immunology
Test background

The complement cascade is activated as part of the innate immune response to pathogens. It is also involved in removing immune complexes and apoptotic debris from the circulation.

These assays measure the ability of serum complement components to form the terminal membrane attack complex (MAC) via the classical and the alternative pathway. The MAC forms cytotoxic pores in the membrane of microbes leading to direct killing. These tests are used as a screening test for defects of individual components of the classical, alternative and terminal pathways and in the investigation of immune deficiency. It should not be used for monitoring complement consumption, instead request C3 and C4 levels.

These tests measure formation of the C5b-9 terminal MAC by ELISA.

Clinical Indications

Suspected complement deficiency
Recurrent infections with encapsulated bacteria in the setting of normal immunoglobulins and normal polysaccharide antibody responses. Recurrent Neisseria infections

Reference range

AP50 – 50-125% of normal
CH50/THC – 50-125% of normal
(ranges derived and validated in house)

Sample & container required Serum (RST rust top)
Sample volume 5-10 mL blood (1 mL serum)
Transport storage For external users, separate and freeze immediately. Transport frozen. For internal users, transport to Charing Cross Hospital Immunology ASAP
Turnaround time 21 days