Rationale:

• Because treatment may be lifelong, the diagnosis must be established as definitely as possible before long-term treatment is considered. This definition of gout will avoid use of uric acid-lowering agents in patients with other forms of recurrent acute arthritis. The differential diagnosis of acute gout also includes articular sepsis, which must be excluded.

Evidence:

• In the American College of Rheumatology committee report for classification of acute gout, examination of fluid obtained by aspiration of the acutely inflamed joint was 84.4% sensitive and 100% specific for the diagnosis (12).
• One survey of hospital laboratories showed that 39 of 50 laboratories could identify monosodium urate crystals, but only 6 of 50 correctly identified concomitant crystal-disease crystals (24).
• Evaluation of joint fluid by a rheumatologist may be of more benefit than simple review by a laboratory technician (25).
• Synovial fluid was aspirated from 50 patients with asymptomatic, nontophaceous gout, in whom monosodium urate crystals had been found in synovial fluid from the knees or other joints. Of these patients, 58% had monosodium urate crystals in their knee joints (26).
• Synovial fluid was obtained from previously affected gouty knee or metatarsophalangeal joints in 91 of 101 consecutive patients with gout. All fluid samples from the 43 patients not receiving uric acid-lowering therapies contained monosodium urate crystals, whereas only 34 of 48 fluid samples showed crystals from those patients receiving uric acid-lowering therapies (27).
• See table Differential Diagnosis of Gout.

Comments:

• One should observe the monosodium urate crystals in cells to prevent false-positive readings from extraneous birefringent material on the microscope slide. Inadequate evaluation of fluid may lead to unnecessary joint lavage, arthroscopy, or arthrotomy.