The validation of a diagnostic rule for gout without joint fluid analysis: a prospective study

The gold standard for diagnosing gout is the identification of MSU crystals in joint fluid. In secondary care, the facilities or expertise to analyse joint fluid are not always available and gout is diagnosed clinically. To improve the predictive value of the clinical diagnosis of gout in secondary care, a diagnostic rule developed in primary care could be helpful. The aim of this study was to validate this diagnostic rule in a secondary care population with the gold standard as reference test. Three hundred and ninety patients with monoarthritis were included. The variables of the diagnostic rule (male sex, previous arthritis attack, onset 5.88 mg/dl) were collected and scored. The affected joint was aspirated and joint fluid was analysed for the presence of MSU crystals. In 219 patients (56%) MSU crystals were found. The positive predictive value of a score of ≥8 points was 0.87, the negative predictive value of a score of ≤4 points was 0.95. The area under the receiver operating characteristic curve for the diagnostic rule was 0.86 (95% CI 0.82, 0.89). The Hosmer-Lemeshow goodness-of-fit test showed that the difference between the expected and the observed probability was non-significant (P = 0.64), indicating good agreement. An easy-to-use diagnostic rule for gout developed in primary care shows good performance in secondary care and improves the predictive value of the clinical diagnosis of gout when joint fluid analysis is not available.