To identify factors to consider in the content of classification criteria for gout, three studies were undertaken (Figure 1). First, clinicians with gout expertise and patients with gout identified factors they believed to discriminate gout from other rheumatic diseases in a Delphi exercise.20 (link) Second, we tested items from this Delphi exercise that were agreed to be potentially discriminatory for gout and items from existing classification criteria in a cross-sectional diagnostic study (Study for Updated Gout Classification Criteria: SUGAR).21 (link) In brief, this study comprised 983 consecutive subjects (exceeding the recruitment target of 860) with joint swelling or a subcutaneous nodule within the previous two weeks, either of which were judged to be conceivably due to gout. These subjects were recruited from rheumatology clinics in 16 countries. All subjects were required to have aspiration of the symptomatic joint or nodule, with crystal examination performed by a certified observer,21 (link), 22 and imaging (ultrasound, radiographs). Cases were those who were MSU crystal positive, while controls were those who were MSU negative, irrespective of clinical diagnosis. Analyses in SUGAR were conducted among two-thirds of the sample (derivation dataset, N=653), reserving the other third as the validation dataset for the final criteria (N=330). Third, we conducted a systematic literature review of advanced imaging modalities for classifying gout.23 (link)