Content Validity and Psychometric Validation of an Adapted Version of the Subject Sleep Diary in Prurigo Nodularis

Patients with prurigo nodularis (PN) often experience sleep disturbance. A psychometric evaluation was conducted to assess the suitability of a PN-adapted multi-item subject sleep diary (SSD) for measuring PN-related sleep disturbance.

Content validity of the SSD was evaluated through qualitative interviews with adults with PN (N = 21). Psychometric properties of SSD parameters (sleep onset latency [SOL], wakefulness after sleep onset [WASO], total awake time [TWT], total sleep time [TST], sleep efficiency [SE], PN-related WASO [WASO-PN], number of WASO-PN, and sleep quality/refresh [SQR]) were evaluated using data from adults with moderate-to-severe PN in the phase 3 OLYMPIA trials (NCT04501666 [N = 286], NCT04501679 [N = 274]). The relationship between SSD parameters and the single-item Sleep Disturbance Numerical Rating Scale (SD-NRS) was examined using equipercentile linking.

Most interview participants who responded to cognitive debriefing questions understood the SSD as intended (≥ 80% for each item), confirming content validity. All SSD parameters showed good test-retest reliability. At week 16, all SSD parameters but TST showed moderate-to-strong correlations, in the expected direction, with the SD-NRS, Peak Pruritus NRS (PP-NRS), Average Pruritus (AP) NRS, Dermatology Life Quality Index (DLQI), PN-related pain frequency and intensity, Prurigo Activity and Severity Score (PAS), and/or Investigator's Global Assessment (IGA). Known-groups validity was adequate for all SSD parameters based on the Patient Global Impression of Severity-Sleep Disturbance (PGIS-SD), Patient Global Assessment of Disease (PGAD), PP-NRS, DLQI, and/or IGA. All parameters but TST were responsive to improvements on the PGIS-SD, Patient Global Impression of Change-Sleep Disturbance, PGAD, PP-NRS, AP NRS, PN-related pain intensity, DLQI, and/or IGA. Cross-sectional values or value changes of the SD-NRS were moderately to strongly correlated with SSD parameters, and equipercentile linking analyses revealed non-linear relationships between the measures.

The findings suggest that the SSD is a fit-for-purpose measure that can be used to assess sleep disturbance in moderate-to-severe PN.

NCT04501666, NCT04501679.