Introduction: Despite numerous treatment options for nail lichen planus (NLP), a validated method for measuring the severity of NLP and therapeutic response in clinical trials is absent. The aim of the study was to develop and validate a measurement instrument, Typical Nail Lichen Planus Severity Index (tNLPSI), for typical NLP that could be used in clinical trials. Methods: A total of 48 patients pathologically confirmed with typical NLP were enrolled in this study. Five dermatologists were trained to use the tNLPSI activity scale and the Physician’s Global Assessment (PGA) scale to score samples independently to estimate inter-rater and intra-rater reliability across two sessions. In addition, tNLPSI activity scores were compared with PGA scores to assess the construct validity. Results: The tNLPSI activity scale had excellent internal consistency and inter-rater reliability (Cronbach’s alpha 0.990; ICC = 0.954; 95% CI = 0.930–0.971), and the correlations between the different graders’ scores indicate good consistency (rp = 0.934–0.968). In addition, the tNLPSI activity scale demonstrated high intra-rater reliability (ICC = 0.996; 95% CI = 0.993–0.998), showing good reproducibility. And tNLPSI activity scores and PGA scores showed good construct validity (Spearman’s rho = 0.941 and Spearman’s rho = 0.903–0.935, respectively; p < 0.01). Conclusion: The tNLPSI activity scale was demonstrated to be consistent, reliable, reproducible, and feasible, making it a potential valuable tool for evaluating the treatment response in typical NLP clinical trials.

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