Deposition of intra-articular
calcium pyrophosphate is associated with both aging and
arthropathy; increased concentrations of free
pyrophosphate (PPi) may contribute to such deposition. Free
pyrophosphate and
nucleoside triphosphate pyrophosphatase (
NTPase) were estimated in synovial fluids from 50 subjects with normal knees and from 44 patients with
rheumatoid arthritis, 61 with
pyrophosphate arthropathy, and 59 with
osteoarthritis. For arthropathic knees clinically assessed
inflammation was classified as active or inactive using a summated score of six clinical features. The order of PPi (mumol/l) and
NTPase (mumol PPi/30 min/mg
protein) was
pyrophosphate arthropathy greater than
osteoarthritis greater than
rheumatoid arthritis (median PPi,
NTPase respectively: for
pyrophosphate arthropathy 15.9, 0.45; for
osteoarthritis 9.3, 0.25; for
rheumatoid arthritis 4.4, 0.18), with significant differences between all groups. In
pyrophosphate arthropathy both PPi (mumol/l) and
NTPase (mumol PPi/30 min/mg
protein) were higher than normal (15.9, 0.45 v 8.6, 0.2 respectively), but findings in
osteoarthritis did not differ from normal. The inflammatory state of the knee had a distinct but variable effect on synovial fluid findings in
rheumatoid arthritis and
pyrophosphate arthropathy, but not in
osteoarthritis. There was no correlation of either PPi or
NTPase with age, or between PPi and
NTPase in any group. This study provides in vivo data for synovial fluid PPi and
NTPase. It suggests that factors other than PPi need to be considered in a study of crystal associated
arthropathy. Clinical
inflammation, as well as diagnosis, is important in synovial fluid studies.