Reliability and validity of the Psoriasis Itch Visual Analog Scale in psoriasis vulgaris.

INTRODUCTION

The single-item Psoriasis Itch VAS was developed to measure itch intensity within the last 24 hours in psoriasis vulgaris to assess treatment benefit. Its psychometric properties were explored in two trials.

METHODS

Data from two randomized, parallel-group phase 3 trials with subjects suffering from psoriasis vulgaris on the body (n = 426, 463) were analyzed. Cross-sectional distributional properties and construct validity of the Psoriasis Itch VAS as well as longitudinal test-retest reliability and sensitivity to change of the Psoriasis Itch VAS were investigated. All statistical tests were two-tailed.

RESULTS

Across both trials, acceptable distributional properties were observed. Convergent-validity correlations between the Psoriasis Itch VAS and other patient-reported and clinician-reported outcomes provided strong endorsement for construct validity as did tests of known-groups validity. Longitudinal measurement properties, involving test-retest reliability and sensitivity to change, also offered evidence for the measurement integrity of the Psoriasis Itch VAS.

DISCUSSION: Results from the assessment of validity, reliability, and sensitivity to change support the use of the Psoriasis Itch VAS to measure itch intensity in psoriasis vulgaris. Data from two trials provided evidence that the Psoriasis Itch VAS is well-defined and reliable for measuring itch in psoriasis vulgaris to assess treatment benefit (i.e. therapeutic response).