Platelets in the synovial fluid of patients with rheumatoid arthritis

In a study of synovial fluid from 110 patients with various forms of arthritis, platelets were identified in the synovial fluid of all the 50 rheumatoids, in 18 out of the 25 (72%) with osteoarthritis and in all 35 of those with other forms of inflammatory osteoarthrosis. Identification of platelets by light microscopy was confirmed by electron microscopy. Platelet counts were significantly higher in rheumatoid fluid (mean 14 988/mm3; range 1000–65 000/mm3) compared with fluid from patients with osteoarthrosis (mean 1592/mm3; 0–10 000/mm3). In addition, significantly higher platelet counts were found in the synovial fluid (SF) of inflamed joints. There was a positive correlation between the SF platelet count and the total white cell count, polymorph count, hydrogen ion concentration, knee score, acid phosphatase and 5-nucleotidase activity and a negative correlation with the glucose level. All these factors indicate joint activity. Finally, platelet numbers correlated with SF levels of immunoglobulin M, and seropositive patients had significantly higher platelet counts in the SF compared with seronegative patients. Rheumatoid patients with thrombocytosis also had higher SF platelet counts. The close relationship of the SF platelet count to other indices of inflammation supports the concept that platelets may directly contribute to synovial inflammation by a variety of pathways.