Radiometric ligand binding assay for C-reactive protein. Complexed C-reactive protein is not detectable in acute phase serum.

A radiometric ligand binding assay for human C-reactive protein (CRP) was established using pneumococcal C polysaccharide (CPS) coupled to magnetizable cellulose particles as the solid phase ligand. Competition for binding to the solid phase between 125I-CRP and unlabelled CRP permitted detection of 30 micrograms/l of CRP and the precise assay of concentrations up to 3000 micrograms/l. Identical results were obtained when the assay was used to quantitate isolated pure CRP and pure CRP added to normal human serum. However in vitro addition of known ligands for CRP to acute phase serum resulted in lowering of the apparent CRP concentration in this assay and addition of as little as 1 microgram/l of free CPS or 1 mg/l of lecithin was demonstrable in this way. A combination of the ligand binding assay and the standard electroimmunoassay for CRP was therefore used to test acute phase sera for the presence of CRP complexed in vitro. No evidence of complexed CRP was detected among sera containing between 1-319 mg/l of CRP from patients with Hodgkin's disease (10), rheumatoid arthritis (10), Crohn's disease (19) and various microbial infections (11), including six with subacute bacterial endocarditis. Since it is likely that CRP does form complexes with its ligands in the plasma these results suggest that complexed CRP is rapidly cleared from the circulation.