Classification of joint fluid based on synovial fluid characteristics

WBC: white blood cell; RBC: red blood cell; CPPD: calcium pyrophosphate dihydrate deposition.
* With most bacterial organisms, particularly Staphylococcus aureus, the synovial fluid WBC count is typically >50,000 cells/microL (and often >100,000 cells/microL). However, lower counts (in the inflammatory range) may be observed in the setting of septic arthritis, especially for disseminated gonococcal infection or if antibiotics were administered prior to joint fluid sampling.
¶ Hemorrhagic synovial fluid usually has <50% neutrophils; however, in some cases a higher proportion may be observed, reflecting the peripheral neutrophil count.
Δ Inflammatory findings are typically observed in the setting of rheumatologic conditions and crystal-induced arthritis; however, concomitant infection also warrants consideration, and synovial fluid culture should be obtained routinely during synovial fluid analysis. In the setting of noninfectious inflammatory arthritis, synovial fluid leukocyte counts may be >20,000 cells/microL (often termed "pseudoseptic"). In general, the higher the synovial fluid leukocyte count, the greater the concern for septic arthritis.