Gout may be diagnosed and treated without further investigations in someone with hyperuricemia and the classic podagra. Synovial fluid analysis should be done, however, if the diagnosis is in doubt. X-rays, while useful for identifying chronic gout, have little utility in acute attacks.
Synovial fluid A definitive diagnosis of gout is based upon the identification of monosodium urate crystals in synovial fluid or a tophus. All synovial fluid samples obtained from undiagnosed inflamed joints should be examined for these crystals. Under polarized light microscopy, they have a needle-like morphology and strong negative birefringence. This test is difficult to perform, and often requires a trained observer. The fluid must also be examined relatively quickly after aspiration, as temperature and pH affect their solubility.
Blood tests Hyperuricemia is a classic feature of gout, but it occurs nearly half of the time without hyperuricemia, and most people with raised uric acid levels never develop gout. Thus, the diagnostic utility of measuring uric acid level is limited. Hyperuricemia is defined as a plasma urate level greater than 420 ?mol/l (7.0 mg/dl) in males and 360 ?mol/l (6.0 mg/dl) in females. Other blood tests commonly performed are white blood cell count, electrolytes, renal function, and erythrocyte sedimentation rate (ESR). However, both the white blood cells and ESR may be elevated due to gout in the absence of infection. A white blood cell count as high as 40.0×109/l (40,000/mm3) has been documented.
Differential diagnosis The most important differential diagnosis in gout is septic arthritis. This should be considered in those with signs of infection or those who do not improve with treatment. To help with diagnosis, a synovial fluid Gram stain and culture may be performed. Other conditions that look similar include pseudogout and rheumatoid arthritis. Gouty tophi, in particular when not located in a joint, can be mistaken for basal cell carcinoma, or other neoplasms.