Synovial fluid osmolality in osteoarthritis and rheumatoid arthritis.

The osmolality of synovial fluid aspirated from the knees of 15 rheumatoid and 15 osteoarthritic patients was observed concurrently with the same measurement on blood serum. The group averages were compared to a group of 13 with identical apparati. Synovial fluid osmolality in rheumatoid arthritics (280 +/- 7.7 mmol/kg) was significantly different from that in osteoarthritics (297 +/- 16.9 mmol/kg), and both group averages were significantly different from normal (404 +/- 57 mmol/kg). Blood serum osmolality also differed between the normal and the two arthritic groups. The potential application of vapor pressure osmometry techniques in the diagnosis and management of joint disease is discussed.