OBJECTIVES

To evaluate the psychometric properties of the Psoriasis Symptoms Scale (PSS), a patient reported questionnaire that includes assessment of worst-level severity of itch, pain, burning, and stinging due to psoriasis in the past 24 hours. The symptom items are summed for a total symptoms score.

METHODS

The PSS symptoms domain was evaluated for floor and ceiling effects, reliability, construct validity, and responsiveness using data collected over 16 weeks from a phase 2 randomized clinical study in patients with moderate to severe plaque psoriasis.

RESULTS

A total of 205 patients were included in the analysis set. There were no observed floor or ceiling effects. The Cronbach’s alpha estimate of 0.84 demonstrated acceptable internal consistency. Test-retest reliability was confirmed in stable patients between screening and baseline visits (7 to 28 days apart) with an intra-class coefficient of 0.86. At baseline and week 16 there were large (>0.5) correlations between the PSS symptoms score and the Dermatology Life Quality Index total and symptoms and feelings domain scores indicating convergent validity; and small (<0.3) correlations with the Short Form-36 mental component summary score indicating discriminant validity. The baseline mean PSS symptoms score was significantly lower (p<.001) in the subgroup with Patient Global Assessment (PatGA) scores ≤ median (PatGA score = 4) compared to those with PatGA scores > median, indicating known-groups validity. The PSS symptoms score demonstrated responsiveness at Week 16 to change in both the Psoriasis Area and Severity Index (PASI) and PatGA, with significantly greater reductions in PSS symptoms score in those achieving any PatGA score improvement and 75% improvement in PASI score compared to those with no or lesser improvements, respectively (p<.001).

CONCLUSIONS

The PSS symptoms score is a reliable and valid measure of patient rated symptom severity due to psoriasis. Interpretation of improvements following treatment will be evaluated next.