Psychometric properties of three single-item pain scales in patients with rheumatoid arthritis seen during routine clinical care: a comparative perspective on construct validity, reproducibility and internal responsiveness

OBJECTIVE To investigate the construct validity, reproducibility (ie, retest reliability) and internal responsiveness to treatment change of common single-item scales measuring overall pain in patients with rheumatoid arthritis (RA) and to investigate the corresponding effect of common pain-related comorbidities and medical consultation on these outcomes. METHODS 236 patients with RA completed a set of questionnaires including a visual analogue scale (VAS), a numerical rating scale (NRS) and a verbal rating scale (VRS) measuring overall pain before and immediately after routine medical consultation as well as 1 week after the patient's visit. Construct validity and retest reliability were evaluated using the Bravais-Pearson correlation while standardised response means (SRM) were calculated for evaluating internal responsiveness. Differences in the perception of pain were calculated using dependent samples t-tests. RESULTS In the total sample, construct validity was good across all three time points (convergent validity of pain scales: rT1-T3=0.82-0.92, p<0.001; discriminant validity as correlation of pain scales with age: rage=0.01-0.16, p>0.05). In patients maintaining antirheumatic treatment, retest reliability of pain scales was confirmed for all scales and across time points (rVAS=0.82-0.95, rNRS=0.89-0.98, rVRS=0.80-0.90, p<0.001), while the internal responsiveness of scales to a change in treatment was low across all scales (SRM=0.08-0.21). The VAS especially suggested a change in pain perception after medical consultation in patients maintaining therapy. CONCLUSIONS The VAS, NRS and VRS are valid and retest reliable in an outpatient clinical practice setting. The low pain scales' internal responsiveness to treatment change is likely to be due to the short follow-up period. Patients with RA maintaining antirheumatic therapy seem to experience less pain after medical consultation.