Guide to interpretation of synovial fluid analysis

WBC: white blood cell; MSU: monosodium urate; CPPD: calcium pyrophosphate crystal deposition.
* Septic arthritis is typically associated with synovial fluid white blood cell counts >20,000 cells/microL, but lower counts may be observed, especially for arthritis due to disseminated gonococcal infection. With most bacterial organisms, particularly Staphylococcus aureus, the synovial fluid white blood cell count is typically >50,000 cells/microL (and often >100,000 cells/microL).
¶ Crystal-induced arthritis may still be considered despite the absence of identified crystals; false-negative results occur, especially with CPPD.
Δ If treatment of crystal-induced arthritis does not result in clinical improvement, consider other inflammatory or infectious arthridites.