A microplate adaptation of the solid-phase C1q immune complex assay

A method has been developed for the detection of C1q binding immune complexes in serum in which microculture plates are used as the solid-phase matrix for adsorption of C1q. This micromethod used only one-tenth of the amount of both C1q and ¹²⁵I]anti-human immunoglobulin per test and enabled 7 times as many samples to be tested in triplicate in comparison with the number performed in duplicate by the standard tube assay.¹²⁵I-labelled C1q studies showed that adsorption varied with the brand of microplate used, some types of plate binding up to 66% of the labelled material. This is considerably more than that bound by the polystyrene tubes generally used for this assay.The increased capacity of the method allowed the binding to plates coated with the same batch of C1q to be assessed at various times after storage for 8–10 weeks at both 4°C and ?70°C. A marked decrease in binding to C1q of aggregates of IgG was observed on storage for longer periods. Results with different batches of aggregated IgG which had been ultracentrifuged suggested that this may be used as an effective standard, stable on storage at ?70°C.A comparison with the standard tube method of aggregated IgG, normal control sera and sera from patients with SLE showed that the micromethod adaptation of the C1q solid-phase binding radioimmunoassay is more economical and easier to perform and does not impair accuracy or standardization.