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The story behind white cell count in the synovial fluid
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Analysis of the synovial fluid plays an important role in the evaluation of joint disease and in particular the diagnosis of infection. For decades the orthopedic community has used the white cell count and neutrophil differential of the synovial fluid for diagnosis of septic arthritis and periprosthetic joint infection. The story behind the investigation of synovial fluid is an interesting one.
Javad Parvizi
Alisina Shahi
The importance of cell count in the synovial fluid was first recognized by Warren and colleagues in 1935. In 1939, Ropes and colleagues performed one of the early investigations on synovial fluid. They conducted an analysis of the cell count on the arterial blood and synovial fluid samples of young cattle. A year later, using the same methodology, the authors performed another study that examined the properties of human synovial fluid. Synovial fluid samples were obtained right after the death of patients who had no signs of joint disease and investigators then evaluated the samples using numerous primitive analytical methods. They did not measure the cell count of the synovial fluid. The analysis of the cell count of the synovial fluid was performed by Coggeshall and colleagues. These investigators aspirated synovial fluid from 29 normal knee joints. The analysis was performed within 30 minutes of the aspiration. Utilizing a hemocytometer, cells in the synovial fluid were counted manually and differentials were determined based on the supra-vital technique. Applying cresyl blue or Unna’s stain, the nucleated cells were separated from red blood cells. No inter- or intra-observer reliability tests were done.
Since then, the medical community has benefitted from the diagnostic features of synovial fluid analysis. According to the guidelines of the American College of Rheumatology (ACR), a joint is considered non-inflammatory when the synovial fluid has fewer than 2,000 cells/mm3. The ACR’s recommendations for cell count have not changed since 1996. Interestingly, the recommendations were based mostly on a study by Shmerling and colleagues that evaluated the diagnostic features of synovial fluid. In the latter study, the synovial white blood cell count threshold for inflammatory arthritis was based on reviews of traditional rheumatology textbooks and reports of synovial fluid evaluations in rheumatic disease. None of these studies were performed to determine the properties of synovial fluid in a native joint.
The reliance on synovial cell count for diagnosis of septic arthritis appears to be based on thin science, if any at all.
References:
American College of Rheumatology Ad Hoc Committee on Clinical Guidelines. Arthritis Rheum. 1996;39(1):1–8.
Coggeshall HC. Anat Rec. 1940. 1;77(2):129–144.
Ropes MW. J Clin Invest. 1939;18(3):351–372.
Ropes MW. J Clin Invest. 1940;19(6):795–799.
Shmerling RH. JAMA. 1990;doi:10.1001/jama.1990.03450080095039.
Warren CF. Am J Pathol. 1935;11(6):953–968.
For more information:
Alisina Shahi, MD; and Javad Parvizi, MD, FRCS, can be reached at the Rothman Institute, 925 Chestnut St., 5th Fl., Philadelphia, PA 19107; Shahi’s email: ali.sinair@gmail.com; Parvizi’s email: parvj@aol.com.
Disclosures: Shahi has no relevant financial disclosures. Parvizi is a consultant to Zimmer, Smith & Nephew, 3M and Convatec.