The modified Nail Psoriasis Severity Index: validation of an instrument to assess psoriatic nail involvement in patients with psoriatic arthritis.

Patients with psoriasis and psoriatic arthritis (PsA) commonly have nail involvement. There is no validated psoriatic nail assessment tool. Recently, investigators developed the Nail Psoriasis Severity Index (NAPSI). Beginning with NAPSI, our goal was to validate a psoriatic nail assessment tool for use in clinical trials, and investigate correlations between nail and other PsA features.Fingernails of 29 patients with PsA were photographed and scored. Clinical data were collected. Using the original NAPSI, analysis revealed high interrater variability of nail scores. Twenty patients' photographs were regraded using the modified NAPSI (mNAPSI).The mNAPSI scores had excellent interrater reliability (Cronbach's alpha 0.98). Nail scores and physicians' global nail severity visual analog scores showed good inter- and intrarater correlations (Spearman's rho 0.85 and 0.90-0.99, respectively; p < 0.01). Several clinical measures correlated with nail scores.The mNAPSI demonstrated excellent interrater reliability and construct validity, and may be a valuable tool.