Worst itch numerical rating scale for prurigo nodularis: a psychometric evaluation

Area of Science:

• Dermatology
• Clinical Trials
• Psychometrics

Background:

• Prurigo nodularis (PN) is a chronic inflammatory skin condition characterized by intense itching.
• Study TR03 investigated nalbuphine extended-release (ER) for PN treatment (NCT02174419).

Purpose of the Study:

• To perform supplementary analyses on the psychometric properties of the Worst Itch Numeric Rating Scale (WI-NRS).
• To evaluate the WI-NRS as the primary endpoint in the TR03 trial for prurigo nodularis.

Main Methods:

• The study involved a double-blind, placebo-controlled, phase 2 trial (TR03) with PN patients.
• Psychometric properties of the WI-NRS, including reliability, validity, and responsiveness, were assessed using TR03 data.
• A responder threshold for the WI-NRS was estimated to aid in interpreting score changes.

Main Results:

• The WI-NRS demonstrated strong test-retest reliability (ICC=0.96) and construct validity with high correlations to other itch measures.
• The scale effectively discriminated between severity subgroups and detected changes in itching severity (effect size=-2.05).
• An estimated responder threshold of ≥3.8 points (approx. 40% improvement) was identified based on Itch VRS improvement.

Conclusions:

• The Worst Itch Numeric Rating Scale (WI-NRS) exhibits robust measurement properties.
• These findings support the use of the WI-NRS for evaluating treatment-induced changes in prurigo nodularis symptoms.