The content validity of the PSS in patients with plaque psoriasis

BACKGROUND: The primary objective of this study was to evaluate the content validity of the Psoriasis Symptom Scale (PSS), with a specific focus on understanding of the content of the PRO measure by conducting one-on-one interviews with patients with moderate to severe plaque psoriasis. This was a cross-sectional, qualitative study conducted with 20 patients with plaque psoriasis who participated in in-person, one-on-one interviews. Participants were asked to describe their psoriasis symptoms, completed the PSS, and were cognitively debriefed on its content. Interviews were conducted in two separate rounds. Following Round 1, the study data were examined to determine if modifications to the PSS were required. All interviews were audio-recorded and transcribed. Sociodemographic and clinical data were collected for sample descriptive purposes. RESULTS: The 20 study participants had a mean age of 50.2 ± 12.0 years (range: 25.0–73.0), and 55% were female. Thirty-five percent of the sample reported their psoriasis severity as moderate or severe. The average time since diagnosis of plaque psoriasis was almost 18 years, ranging from less than one to over 38 years. The most frequently reported symptoms and signs during the concept elicitation portion of the interviews included redness (N = 20, 100%), itching (n = 20, 100%), pain (n = 15, 75%), burning (n = 13, 65%), and flaking (n = 11, 55%). Overall, participants provided positive feedback on the PSS and felt that it was comprehensive and relevant to their experience with psoriasis. The item meaning and response options were well-understood for the majority of the items. Findings indicate that for the patient-reported symptom of redness, which is also a sign that can be reported by clinicians, redness or the perception of redness is most accurately captured by patient report. Study results did not support modifications to the instrument and no changes to the PSS were recommended. CONCLUSION: The evidence gained in this study provided support for the content validity of the PSS for use as clinical trial endpoint among patients with plaque psoriasis. This study found that the symptoms included in the PSS are important to and well-understood by patients with plaque psoriasis. The PSS is appropriate for inclusion in future studies designed to measure the effect of treatment on psoriasis-related symptoms.