For synovial fluid analysis. WBC level more than 50,000 (especially if more than 80,000) is suggestive of septic arthritis.
RBC count is to exclude traumatic tap, usually the ratio is about 500 RBC: 1 WBC.
Differential, normally, neutrophils account for less than 25% of the WBC in the synovial fluids. Ratio more than 75% (especially > 90%) is suggestive of septic arthritis.
Normally synovial fluid is transudate with <200 WBCs and <25% neutrophils.
LDH level = 310 IU/L. (normal level 105-333, depending on the lab). LDH might be at elevated levels in people who have rheumatoid arthritis, infectious arthritis, or gout.
The presence of urate crystals indicates a diagnosis of gout.
Although, this patient had uric acid crystal in the aspirated synovial fluid with no bacteria on microscopy (Gram stain), The level of WBC and their differential were concerning, This patient was taken to the theatre and it was confirmed that he had both gout and septic arthritis.
The rate of finding bacteria on microscopy testing in septic arthritis is 60-80%. (Less than that in gonorrheal arthritis).
Septic arthritis can be a complication of gout.
Synovial fluid white blood cell count and percentage of polymorphonuclear cells perform well as discriminators between inflammatory and noninflammatory disease. Ordering chemistry studies of synovial fluid should be discouraged because they are likely to provide misleading or redundant information.