Synovial lactic acid and septic arthritis.

To the Editor: In their Rational Clinical Examination article, Dr Margaretten and colleagues concluded that the synovial fluid white blood cell count and the corresponding percentage of polymorphonuclear cells are the most useful markers in identifying septic arthritis while waiting for the Gram stain and culture test results. Synovial fluid glucose, protein, and lactate dehydrogenase were not found to be helpful. In the past, synovial fluid lactic acid measurement was proposed as a useful test in the rapid differentiation between septic and nonseptic arthritis. For example, in the study by Riordan et al, lactic acid seemed to be more sensitive than Gram stain (especially if antibiotics had been administered before joint aspiration) and could be measured even when the synovial fluid was too thick for a cell count to be performed. In addition, synovial fluid lactic acid can be assessed rapidly in a blood gas analyzer and may be available to clinicians even before the synovial fluid cell count and differential. It would be helpful if the authors’ literature search provided data on the value of this easy-toobtain and seemingly useful test.