Q: 46 year old female with history of gout is admitted to ICU with fever, hypotension and right swollen knee with pain. Knee is aspirated under sterile condition. You received call from lab that Lactate level in joint aspirate is 16 mmol/l. What does it mean?

Answer:  Likely diagnosis of septic arthritis instead of inflammatory process 

A lactate level in the synovial fluid of greater than 10 mmol/l makes the diagnosis very likely. synovial lactate is a useful adjunct test to synovial WBC, gram stain and other values in ruling in/out septic arthritis.

Reference:

Carpenter, CR; Schuur, JD; Everett, WW; Pines, JM (August 2011). "Evidence-based diagnostics: adult septic arthritis.". Academic emergency medicine : official journal of the Society for Academic Emergency Medicine 18 (8): 781–96