To the Editor:
It was with great interest that we read the letter from Watson and colleagues1 describing their investigation of how a primary care diagnosis of gout compares to a primary care diagnostic rule for gout and the 1977 American Rheumatism Association (ARA) classification criteria of acute arthritis of primary gout. They present positive predictive values (PPV) of 74% for the diagnostic rule and 80% for the ARA criteria. We were not surprised by the vast lack of urate values. Almost 47% did not have a serum urate level recorded in their medical record. It is troublesome that this mandatory factor for development of gout is not controlled in almost half of the cases. Further, we noticed that presence of tophi was quite low in the population (only 2.4%), but this may reflect the medical records in primary care rather than the actual prevalence, which would be considerably higher. The lack of urate levels and the low prevalence of tophus in the medical records lead to an underestimation of the PPV, as the authors also mention. Thus, we must concur that the vast majority of gout cases in primary care fulfill classification criteria and are valid for research purposes2.