Functional complement assays for the classical pathway (CH100) or alternative pathway (AP100) are only of value in detecting genetic deficiencies in individual components. They have no role in monitoring complement activation per se. C3 and C4 measurements should be used to investigate complement activation. Complement functional assays will require specific sample types and also specific transport arrangements. Genetic deficiencies of C3, C5 or alternative pathway are usually associated with recurrent bacterial infection; deficiencies in the classical pathway are usually associated with immune complex disease. Deficiencies of proteins which control the alternative pathway are more common than previously thought and are associated with atypical haemolytic uraemic syndrome. The CH100 and AP100 assays identify which component is likely to be absent.
Specific component assays can then be carried out. The laboratory will advise on these complex assays.
Screening tests for complement pathway deficiencies in patients with clinical history suggesting high probability of such deficiencies. The clinical symptoms (including type of infectious organisms concerned in repeated infections) give a strong indication of likely deficiencies.
Taken with C3, C4 and C1inhibitor assays, abnormal functional CH100 and AP100 assays may provide an indication that further investigations are warranted to identify specific deficiencies. Some component deficiencies (being recessive) are exceedingly rare and usually found only in consanguineous relationships. Others are more common and non-consanguineous e.g. deficiencies in classical pathway components which are usually associated with SLE and related connective tissue disease.
Serum Separator Tube (SST)
Referred test only performed after consultation with the laboratory. Sample must be received in laboratory as soon as possible certainly within 3 hrs of venepuncture.
70-100% level of normal sample tested on that day.