Interference caused by the contents of serum separator tubes in the Vitros CRP assay

Background We have observed discrepancies between C-reactive protein (CRP) measured in serum prepared from a serum separator tube (SST) and that obtained from a plain tube, when using the Vitros CRP assay. Our study aimed at elucidating the cause of these discrepancies. Methods Eighty-seven specimens from hospitalized patients with various types of inflammatory disease were analysed using a fixed-point immuno-rate method on a Vitros CRP slide. The serum was prepared simultaneously in both vacuum and SSTs. We also performed mixing tests by adding 47 samples of serum prepared from plain tubes to SSTs and incubating for 15 min before CRP analysis. Results Lower values of CRP were found in serum prepared from plain tubes than in serum from SSTs. Addition of serum prepared from plain tubes to SSTs and incubating for 15 min increased the CRP values significantly. The ratio of CRP measured in serum prepared from plain tubes and from SSTs did not differ significantly from the ratio obtained when serum was prepared in a plain tube then added to an SST. Discussion We propose that SSTs can adsorb some macromolocules that form complexes with CRP. The addition of SST gel to serum results in the release of CRP molecules from these complexes, which enhances the antigen-anti body reaction on the Vitros CRP slide and increases the measured CRP concentrations.