Abstract
The nail is an important characteristic in the context of psoriasis. Nail psoriasis exhibits several clinical manifestations. Since the development of the Nail Psoriasis Severity Index (NAPSI) score assessment tool for evaluating the severity of nail psoriasis, nail matrix symptoms such as pitting, leukonychia, crumbling, and red spots have been observed in the lunula. Nail bed symptoms include onycholysis, subungual hyperkeratosis, oil spots, and splinter hemorrhage. However, Beau's lines and nail fold psoriasis, which are not included in this assessment tool, should be considered essential symptoms for indicating the activity of nail psoriasis. Although NAPSI is the most widely used tool for assessing the severity of nail psoriasis, it has uncontrolled limitations. Although other assessment tools have been developed, none have successfully replaced the NAPSI. In clinical trials for nail psoriasis, the NAPSI is used in various forms, such as improvement rates, changes in the mean NAPSI score, and achievement rates of NAPSI 75. Consequently, caution is warranted when interpreting the clinical trial results related to nail psoriasis improvement.