When trying to diagnosis a septic joint, it is common to order the following labs on the synovial fluid:
Synovial WBC count
Glucose
Protein
Gram Stain
Culture
Unfortunately, there is no value of glucose or protein that has enough sensitivity and specificity to make the tests diagnostically helpful. Gram stains are only postive in culture positive septic joints in approximately 50% of the cases. Cultures take too long to be helpful in the ED. The synovial WBC count can be helpful if very high, but a low value does not ensure that the patient does not have a septic joint.
The one test that has been shown to have a Positive Likelihood ratio of Infinity is a synovial lactate level >10. A synovial lactate should be sent on all synovial fluid as a level of 10 and greater makes the diagnosis of septic arthritis, regardless of the gram stain or synovial WBC level.
