Serum and synovial fluid histidine: a comparison in rheumatoid arthritis and osteoarthritis.

The serum and synovial fluid (SF) histidine, sulphydryl, and protein concentrations were compared in simultaneous samples from 84 patients with rheumatoid arthritis (RA) and a control group comprising 29 patients with osteoarthritis (OA). The SF levels of histidine were higher than the serum levels in the RA patients but significantly lower than corresponding results in patients with OA (P<0.001). The latter had levels of serum and SF histidine which were equivalent and within the normal range. Greater quantities of protein were found in the SF of the patients with RA compared with the OA group. The serum and SF sulphydryl concentrations expressed as μmol/g protein were low but in equilibrium in patients with RA. However the SF sulphydryl (μmol/g protein) was depressed relative to serum levels in patients with OA.