Validation of the labeled magnitude scale for the assessment of itch intensity in patients with chronic pruritus

Introduction: The visual analogue scale (VAS), the numerical rating scale (NRS), and the verbal rating scale (VRS) are routinely used to assess pruritus intensity. However, these scales have limitations, especially the ceiling effect of linear scales (VAS, NRS) and the reduced sensitivity to change of the VRS. In the labeled magnitude scale (LMS) consisting of a vertical line with verbal anchors distributed quasi-logarithmically, the ceiling effect is reduced, allowing a higher discriminative assessment at the higher end of the scale. Methods: A total of 172 patients completed the LMS twice within 30–60 minutes to assess the reliability of the scale, as well as the NRS and VAS for analyses of convergent validity. Sensitivity to change of the LMS was investigated by analyzing changes in scores after a twice-daily application of an emollient containing menthoxypropanediol for 14 days. Results: Test-retest reliability was excellent for the LMS assessing average (Cronbach α: 0.955) and worst (Cronbach α: 0.945) pruritus intensity in the last 24 hours, while strong to very strong correlations were observed between the LMS and NRS assessing the worst (r=0.783) and average (r=0.808) pruritus intensity, respectively. Treatment with an emollient lead to a significant decrease in LMS scores (P<0.01). In patients with severe pruritus at baseline (VRS ≥3), we recorded a 30.2% improvement of average pruritus intensity using the LMS compared with a 25.0% improvement using the NRS and a 27.8% improvement of worst pruritus intensity using the LMS compared with an 11.1% improvement with the NRS. Most patients considered the LMS an appropriate instrument to assess pruritus intensity (89.6%) and would use it again (91.9%). Discussion: The LMS is an appropriate well-accepted instrument to assess CP. It is especially useful to detect variations in pruritus intensity in patients with severe CP.