The Nail Psoriasis Severity Index (NAPSI) is a physician-reported instrument for grading the severity of nail psoriasis, published in 2003 by Rich and Scher. Nail involvement affects up to half of patients with psoriasis and up to 80% of patients with psoriatic arthritis, and it is often painful, functionally limiting, and slow to respond to treatment. The NAPSI was designed as a reproducible, numeric measure of nail disease that could be used to compare therapies in clinical trials.

Each nail is divided into four quadrants by imaginary horizontal and longitudinal lines. Signs of nail matrix psoriasis (pitting, leukonychia, red spots in the lunula, and nail plate crumbling) and signs of nail bed psoriasis (onycholysis, splinter haemorrhages, oil drop or salmon patch discolouration, and nail bed hyperkeratosis) are assessed separately. For each nail, the matrix score is the number of quadrants (0 to 4) in which any matrix feature is present, and the nail bed score is the number of quadrants (0 to 4) in which any nail bed feature is present. Multiple features of the same category within one quadrant count once. Each nail therefore scores 0 to 8, the 10 fingernails give a total of 0 to 80, and inclusion of the toenails extends the range to 0 to 160.

Several variants are described. The original paper also allows each of the eight features to be scored individually per quadrant, giving a more detailed 0 to 32 score for a single target nail, and many trials follow only the most severely affected target nail (0 to 8). The modified NAPSI (mNAPSI) of Cassell et al. (2007) grades pitting, crumbling, and combined onycholysis and oil drop change from 0 to 3 and records four further features as present or absent, giving 0 to 13 per nail; it showed higher interrater reliability than the original but is applied inconsistently across studies. This calculator implements the original NAPSI.

No validated severity thresholds or minimal clinically important difference exist for the NAPSI, and expert consensus acknowledges that no ideal nail psoriasis severity index is currently available. Interobserver reliability of the total score is acceptable (intraclass correlation coefficient 0.78), with nail bed features scored more consistently than matrix features, although reliability is lower among untrained assessors. In clinical trials the NAPSI is the most frequently reported nail outcome, usually expressed as mean change or percent improvement from baseline, with NAPSI 50, NAPSI 75, and NAPSI 90 denoting improvements of at least 50%, 75%, and 90%. Because nails grow slowly, meaningful change typically requires 6 to 12 months of treatment.