Hemolytic and antigenic measurements of complement. A comparison of serum and plasma samples in normal individuals and patients.

Over a 2-year period we evaluated 10 patients with a discrepancy between functional and antigenic assays in the routinely employed clinical assays for measuring serum complement concentrations. These differences were shown to be secondary to cold-dependent activation of the classical complement pathway in vitro, in some cases by cryoglobulins and in others by unknown means. Plasma samples were procured with commonly used anticoagulants (EDTA, citrate, or heparin) that prevent in vitro complement activation. To assess whether plasma samples were suitable for complement determination, a comparison of serum vs. plasma samples for THC, C4, C2, C3, factor B, and C6 levels in normals and patient populations was undertaken. Only modest differences were found between serum and plasma samples for these functional and antigenic assays except for heparinized samples in which determinations by rate nephelometry produced falsely elevated C4, C3, and factor B antigenic levels. Thus plasma samples, especially EDTA or citrate, are suitable for complement determinations and could be used either routinely or, more selectively, in patients in whom there is a discrepancy between functional and antigenic determinations.