Development of a rapid kinetic assay for the function of the classical pathway of the complement system and for C2 and C4.

A simple functional complement assay (FCA) is described which is based on kinetic turbidimetry. 25 microliter of sample are combined with 250 microliter of a suspension of antibody coated ovine erythrocytes in a microprocessor controlled photometer. After a lag phase the activation of complement leads to a rapid lysis of the cells which is measured photometrically at 578 nm. The time for a decrease of absorbance of 0.1 is defined as the endpoint of the reaction. Normal citrated plasma which is the preferred specimen for the assay shows a reaction time of about 42 sec whereas pathological or diluted samples show a prolongation. With specific antibodies it was demonstrated that the assay was sensitive for a deficiency of factors of the classical pathway. Factors of the alternative pathway do not influence the assay. In combination with deficient sera a modified form of the assay allows also the determination of C2 and C4. Because of its simplicity, speed and accuracy these assays may be well suited to perform functional complement assays in a routine laboratory.