Clinical prediction rules effectively diagnose gout without joint fluid analysis. The American College of Rheumatology clinical prediction rules, the most accurate rules developed for research purposes, have a sensitivity of 92%, specificity of 89%, positive likelihood ratio of 8.36, and negative likelihood ratio of 0.09 (strength of recommendation [SOR]: A, prospective cohort studies).
The Netherlands criteria, developed for use in primary care, have a positive predictive value of more than 80%, a positive likelihood ratio of 3.48, and a negative likelihood ratio of 0.17 (SOR: A, prospective cohort study).
In 2015, the American College of Rheumatology (ACR) redefined the clinical criteria for diagnosis of gout based on a 3-step system1 that can be found at: http://goutclassificationcalculator.auckland.ac.nz. The ACR rule was derived from a cross-sectional study of 983 patients in 25 rheumatology centers in 16 countries who presented with a swollen joint.2 Of the 983 patients, 509 had gout; the prevalence was 52%. Data from 653 of these patients were used to develop the rule and then validated in the remaining 330 patients.
Compared with the gold standard of monosodium urate crystals in synovial fluid, the ACR rule has a sensitivity of 92% and a specificity of 89%. The rule, designed for the research setting, involves using synovial fluid analysis, ultrasound imaging, and radiography, which makes it less useful in a primary care setting.
The Netherlands rule for primary care
A prospective diagnostic study in 328 family medicine patients (74% male; mean age 57) with monoarthritis tested the ability of multiple clinical variables to diagnose gout using monosodium urate crystals in synovial fluid as the gold standard.3 The prevalence of gout in this population was 57%.
The best diagnostic rule (Netherlands rule) comprised the following predefined variables: male sex, previous patient-reported arthritis attack, onset within one day, joint redness, first metatarsophalangeal joint (MTP1) involvement, hypertension or cardiovascular disease (angina pectoris, myocardial infarction, heart failure, cerebrovascular accident, transient ischemic attack, or peripheral vascular disease), and serum uric acid level above 5.88 mg/dL. The rule gives one point for each item. A score >8 had a positive likelihood ratio for diagnosing gout of 3.48 (TABLE1) and a higher positive predictive value (PPV) than family physicians’ clinical impressions (83% vs 64%).
The prevalence of gout in patients with scores of <4, 4 to 8, and >8 were 2.8%, 27%, and 80%, respectively. For scores of 4 to 8, the probability of gout is indeterminate, and synovial fluid analysis is recommended.
The Netherlands rule, validated in a secondary care practice of 390 patients with monoarthritis, found that a score >8 had a PPV of 87% and a score <4 had a negative predictive value of 95%.4 The probability of gout based on this rule can be calculated at http://www.umcn.nl/goutcalc.
In the study used to develop the Netherlands rule, no patients with a high probability of gout had septic arthritis. The ability of the rule to differentiate between gout and septic arthritis was tested retrospectively in 33 patients with acute gout (podagra excluded) diagnosed by the presence of monosodium urate joint crystals and 27 patients with septic arthritis diagnosed by positive bacterial culture.5 Patients with gout had significantly higher scores than patients with septic arthritis (7.8 ± 1.59 vs 3.4 ± 2.3; P<.001).