Validation of pain and patient global scales in chronic gout: data from two randomised controlled trials.

Objective: To assess validity of pain and patient global scales in gout.

Methods: The authors used data from pegloticase clinical trials in chronic refractory gout to examine the validity of visual analogue scale (VAS) pain, Short-Form 36 (SF-36) bodily pain subscale and VAS patient global assessment (all scales 0-100). Convergent/divergent validity with clinical characteristics was tested by using Spearman's correlation coefficient. For discriminant ability, the authors compared the change at 6 months between placebo and pegloticase arms and calculated effect size (ES) and standardised response mean (SRM).

Results: 212 patients (mean age, 55.4 years, 82% men; 73% with tophaceous gout) provided data. VAS pain was statistically significantly correlated with tender joints (r=0.42), swollen joints (r=0.30), SF-36 physical (r=-0.56) and Mental Component Summary (r=-0.36) and Health Assessment Questionnaire scores (r=0.54; all p-values <0.0001), but not disease duration (r=-0.01; p=0.84), gout flares (r=0.12; p=0.08), comorbidities (r=0.05; p=0.47) or plasma urate (r=0.01; p=0.89). Similar and significant correlation coefficients with tender and swollen joints were noted for VAS patient global assessment (r=0.35 and 0.23; p<0.0012 for both) and SF-36 pain subscale (r=-0.27 and -0.19; p<0.006 for both). Pegloticase group had significantly more improvement than placebo at 6 months, mean (SD): VAS pain, -9.2 (29.3) versus 1.9 (26.4), p=0.0002; SF-36 pain, 14.6 (25.6) versus -0.04 (21.1), p<0.0001; and patient global, -9.3 (26.5) versus 3.4 (22.8), p<0.0001. ES and SRMs in pegloticase group were as follows: VAS pain, 0.34 and 0.30; SF-36 pain, 0.69 and 0.57; patient global, 0.49 and 0.44.

Conclusions: VAS pain, SF-36 pain and patient global VAS are valid outcome measures in patients with chronic gout.