It is challenging to distinguish nail psoriasis (NP) from nonspecific nail changes, contributing to heterogeneity in clinical trials. Existing scoring tools for NP are currently used to assess severity after diagnosis is established. The aim of this study is to evaluate the diagnostic performance of two of these severity scoring tools. A cohort study was conducted with psoriasis patients and matched controls. Fingernails were scored using the Nail Psoriasis Severity Index (NAPSI) and the Nijmegen-Nail Psoriasis Activity Index Tool (N-NAIL). To determine their diagnostic properties, cutoff values were established. Receiver operating characteristic (ROC) curves were constructed, and sensitivity and specificity were calculated for various cutoff points. The best cutoff value was chosen based on the Youden Index and clinical reasoning. In total, 104 psoriasis patients were included, of which 68 were clinically diagnosed with NP. For the N-NAIL, a cutoff value of 2 showed the best accuracy in the psoriasis population (sensitivity = 83.8% and specificity = 83.3%) and the general population (sensitivity = 83.8% and specificity = 67.3%). For the NAPSI, a cutoff value of 7 showed the best accuracy in the psoriasis population (sensitivity = 80.9% and specificity = 69.4%), while a cutoff value of 10 was optimal in the general population (sensitivity = 72.1% and specificity = 70.2%). Both N-NAIL and NAPSI provide accurate cutoff values in a psoriasis population. Therefore, these scoring tools may not only be used to assess severity but also in clinical trials for the inclusion of NP patients in a psoriasis population to create homogeneity between studies. We prefer using the N-NAIL, with a cutoff value of 2, because it showed better accuracy compared to the NAPSI.
