It is generally taught that if the synovial fluid white blood count (WBC) is less than 50,000 it is not septic, however, there is growing evidence that a clear delineation in the WBC between septic arthritis and inflammatory arthritis is not possible. In fact, inflammatory arthritis (rheumatoid and gout) actually increases your risk for septic arthritis and the two can coexist. Gram stains of the fluid only show organisms in 50% of those with septic arthritis so you also can not rely on them either. Inflammatory markers (CRP, ESR) can be elevated with inflammatory or septic arthritis so they too can not differentiate between the two.
In the end, because of the risk of permanent joint dysfunction, it is important to make the diagnosis on clinical grounds and treat empirically if you are unsure. Err on the sound of treatment. Serial joint aspirations to drain synovial fluid have the same outcomes as operative washout.
A recent article that discusses the concerns with making the diagnosis of septic arthritis is:
