Reliability and validity of the psoriasis symptom inventory in patients with moderate-to-severe psoriasis.

Objectives: The psoriasis symptom inventory (PSI) is a patient-reported outcome measure for assessing symptom severity in patients with moderate-to-severe psoriasis. The primary objective of this study was to evaluate the measurement properties of the PSI.
Materials and Methods: Analyses of psychometric characteristics (reliability, convergent and known-groups validity,responsiveness, item performance, and dimensionality) were conducted using data from a Phase II trail to evaluate efficacy of brodalumab in subjects with moderate-to-severe psoriasis.
Results: The PSI had excellent internal consistency (α = 0.93-0.98) and good test-retest reliability (ICCs = 0.77-0.87). Convergent and discriminant validity was indicated by moderate-to-strong correlations between the PSI and Dermatology Life Quality Index scores, and small correlations between PSI total scores and ShortfFrm-36 Health Survey mental health, role emotional, and role physical scales. Known groups validity was shown as mean PSI total scores varied by Psoriasis Area and Severity Index (PASI) and Static Physician's Global Assessment (sPGA) defined groups (p < 0.001). PSI total scores were responsive to changes in clinical status as assessed by PASI (p < 0.001) and sPGA (p < 0.001). Unidimensionality of the PSI was supported.
Conclusions: The PSI is a short and valid unidimensional measure of psoriasis symptom severity that is well suited for use in clinical trials.