Gout and Hyperuricemia
Diagnosis of Gout
Gout usually presents with characteristic inflamed, painful joints in the extremities, but these symptoms are also common to other conditions, particularly pseudogout (a related condition caused by the accumulation of calcium pyrophosphate crystals in the joint) or septic arthritis (caused by joint infection). Blood tests can determine whether the patient is hyperuricemic (typically a serum concentration above 7 mg/dL in males and above 6 mg/dL in females). While hyperuricemia is the most important risk factor for gout its diagnostic power may be limited; as some hyperuricemic patients may never develop the disease, and blood urate levels may be normal during an acute attack (in one study, 14% of patients had blood uric acid levels of <6 mg/dL during their gout attacks).17
The European League Against Rheumatism (EULAR) recently published guidelines for gout diagnostic criteria, based on accumulated studies of gout diagnoses. According to their analysis, the most definitive feature in the diagnosis of gout is the identification of monosodium urate crystals in synovial (joint) fluid or aspirates of tophi.15 This involves the insertion of a fine needle into the joint or tophus and withdrawing a fluid sample, which is subsequently examined under a microscope.47 The presence of large needle-shaped crystals confirms the presence of monosodium urate, and can be used to differentiate gout from pseudogout or septic arthritis.
The presence of urate crystals in the joints of asymptomatic patients can be used to identify intercritical periods in patients with recurrent gout, or can be used to screen patients who may benefit from urate-lowering therapy prior to their first acute gout attack. Screening of the joints of the extremities using ultrasound is being explored as a non-invasive method to improve the detection of urate crystals in asymptomatic patients. In a pilot study, ultrasound correctly identified urate crystals in the joints of asymptomatic, hyperuricemic volunteers with an accuracy of 81%.48