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-antibody reaction on
the Vitros CRP slide and increases the measured CRP concentrations.
