Complement C3 in serum and plasma, as measured by radial immunodiffusion with four commercial kits.

Quantitation of the C3 component of complement by single radial immunodiffusion is subject to error because the C3 molecule has several antigenic determinants and anti-C3 sera differ in their specificity to these determinants. C3 was measured in plasma (ethylenediaminetetraacetate anticoagulant) and serum, in five normal individuals, by use of four such commercial kits. The effects of ethylenediaminetetraacetate and of incubation at -20, 4, and 37 degrees C for 1, 2, and 7 days were investigated. We found wide variations in C3 in the same sample, as measured with different kits. Incubation for longer than 48 h at 4 degrees C changed C3 concentrations as compared to those in fresh samples. Reference sera for C3 assay and standardized procedures for sample processing are both essential to valid results.