Historically, synovial fluids have been classified as noninflammatory (WBC count 50-1000), inflammatory (WBC count 1000-75,000), septic (WBC count>100,000), or hemorrhagic. However, septic synovial fluid can have cell counts as low as a few thousand early in the infectious process; thus, differentiation of inflammatory and septic joints is not truly possible based on cell counts alone.
When a patient presents with an acute or chronic monoarthritis, infectious arthritis should be considered. In one study of 77 patients with septic arthritis, the peripheral blood leukocyte count was within the reference range in 55% of the patients, and, in 13% of the cases, the erythrocyte sedimentation rate (ESR) was less than 20 mm/h. The diagnostic value of the synovial fluid leukocyte count in an individual case is poor. Counts below 25,000/mm3 are observed commonly in infectious arthritis. Thus, the key diagnostic test when septic arthritis is suggested is arthrocentesis with analysis and culture of synovial fluid.
Patients may present with polyarticular involvement, so the presence of multiple inflamed joints does not rule out infectious arthritis. Polyarticular involvement is the rule in certain types of arthritis, such as gonococcal arthritis and rheumatoid arthritis (RA).
The important message for the physician is to be aggressive in looking for infectious arthritis. If septic arthritis is suggested, aspirate. In skilled hands, the discomfort and morbidity of joint aspiration are minimal. The speed of diagnosis is the most important determinant of the outcome.
Synonyms and related keywords:
synovial fluid, acute monoarthritis, chronic monoarthritis, polyarticular arthritis, infectious arthritis, gonococcal arthritis, rheumatoid arthritis, RA, infection
Author: Nadera Sweiss, MD 2003