1. A 1H-n.m.r. method was used to measure concentrations of valine, alanine, lactate, acetate, hyaluronan and lipids in synovial fluid obtained, during the normal course of examination, from the knee joints of patients attending rheumatology and orthopaedic clinics. Fluid was available from 16 patients with osteoarthritis, 18 patients with rheumatoid arthritis, four patients with meniscal tear and one patient each with systemic lupus erythematosis, mono-arthritis, synovitis and loose bodies. Four normal specimens were obtained for comparison.

2. Valine, alanine and acetate levels all showed a normal Gaussian distribution, reflecting the distributions within the serum of the sample population.

3. Lactate concentrations divided into two distinct patterns. At concentrations below 2.5 mmol/l the lactate levels showed a Gaussian distribution, reflecting the distribution in normal serum. The normal synovial fluid specimens belong to this distribution. Above 2.5-3.0 mmol/l, lactate levels were asymmetric in distribution with a long tail at higher concentrations. These high levels of lactate can be explained by the generation of lactate through anaerobic metabolism within the synovial cavity. This metabolic process is triggered by a general inflammatory condition such as in rheumatoid arthritis.

4. The distribution of n.m.r.-observable lipid concentrations in rheumatoid arthritis and osteoarthritis each shows a normal distribution and the mean concentration is significantly higher in rheumatoid arthritis.

5. An increased n.m.r.-observable hyaluronan concentration is associated with an inflammatory situation.

6. It is concluded that raised levels of lactate and n.m.r.-observable hyaluronan and lipids are useful markers to aid the clinical distinction between rheumatoid arthritis and osteoarthritis. The results of this study confirm rheumatoid arthritis as an inflammatory disease inducing higher metabolic activity within the synovial cavity.

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