To assess validity of pain and patient global scales in gout.
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).
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.
VAS pain, SF-36 pain and patient global VAS are valid outcome measures in patients with chronic gout.