An 80-year-old woman was hospitalized 2 days ago for upper gastrointestinal bleeding due to peptic ulcer disease. She was placed on omeprazole and given intravenous normal saline for hydration. Today she has developed right knee pain and swelling. History is also significant for osteoarthritis of the hands and knees. Her only medication prior to admission was ibuprofen as needed.
On physical examination, temperature is 37.8 °C (100.0 °F); the remainder of the vital signs is normal. Hand findings are consistent with osteoarthritis. The right knee is warm with a large effusion, is tender to palpation, and has limited flexion to 90 degrees.
Aspiration of the right knee is performed; the synovial fluid is yellow and cloudy, and the leukocyte count is 15,000/µL (15 × 109/L).
Which of the following synovial fluid tests will be most helpful in establishing a diagnosis?