N-NAIL Diagnostic Capacity Compared to NAPSI in Patients with Nail Psoriasis

A cutoff score of 2 on the Nijmegen-Nail Psoriasis Activity Index Tool (N-NAIL) is the most accurate nail psoriasis identifier among individuals with plaque psoriasis, new data suggest, outperforming the Nail Psoriasis Severity Index (NAPSI) in diagnostic accuracy.¹

Evi C.C. Rikken—a dermatology resident from Radboud University Medical Center Department of Dermatology in the Netherlands—led a team of other authors in the analysis that led to this conclusion. Rikken et al highlighted these findings among patients who had been recruited from the Discovery of Arthritis in Psoriasis Patients for Early Rheumatological Referral (DAPPER) study.²

The investigators noted that the NAPSI tool is the most widely implemented scoring tool for nail psoriasis, with many different versions of the tool. However, N-NAIL was developed as another validated scoring tool for nail psoriasis.³

“With this study, we aim to investigate if it is possible to define a cutoff value for the N-NAIL and/or NAPSI to distinguish NP from other nonspecific nail changes in patients with psoriasis of the skin and in the general population,” Rikken and colleagues wrote.¹

Trial Design Details

The investigative team conducted this cohort study between March 2021 – May 2023 at the Radboud University Medical Center Department of Dermatology in Nijmegen, the Netherlands. The center is known to serve as both the national referral center for nail disorders and as a psoriasis expertise hub. Eligible individuals to take part in the analysis were patients aged 16 years or older with a clinician-confirmed diagnosis of plaque psoriasis.

The patients were included in the team’s analysis regardless of whether nail psoriasis was present. Part of Rikken and coauthors’ patient population was recruited from the DAPPER study, an analysis designed to assess the prevalence of psoriatic arthritis (PsA) among those with psoriasis and to help identify markers helping clinicians to guide the refinement or creation of referral tools for rheumatology.

The investigators sought reliable comparisons, so trial subjects with psoriasis were matched to controls without psoriasis or nail pathology based sex and age. Over the course of their cohort study, Rikken et al evaluated fingernail involvement through 2 established scoring instruments: NAPSI and N-NAIL. They determined their diagnostic thresholds by calculating receiver operating characteristic (ROC) curves, followed by sensitivity and specificity testing at different cutoff values. Optimal thresholds for the analysis were identified by the investigators through a combination of the Youden Index and clinical judgment.

NAPSI Versus N-NAIL Results

There were 104 individuals with psoriasis who were enrolled in Rikken and colleagues’ research. Among these 104 participants, 68 reported a clinical diagnosis of nail psoriasis. When looking at N-NAIL’s results, the most accurate cutoff in the psoriasis arm was 2, with a sensitivity of 83.8% and a specificity of 83.3% being observed by the investigative team.

The team’s general population comparison maintained the same cutoff of 2, with results being a sensitivity of 83.8% and specificity of 67.3%. Among those assessed via NAPSI, the optimal cutoff within the psoriasis arm was 7 (sensitivity 80.9%, specificity 69.4%), whereas those in the general population the best cutoff shifted to 10 (sensitivity 72.1%, specificity 70.2%).

These data would suggest that both N-NAIL and NAPSI provide clinicians with reliable cutoff values among individuals living with psoriasis and could be applied not only to grade severity but also to standardize patient inclusion criteria in clinical trials on NP. Based on the diagnostic tool’s superior performance, particularly at a cutoff value of 2, Rikken and colleagues concluded that N-NAIL may be preferred over the NAPSI.

“For the future, it is important to aim for a situation where both inclusion and follow-up in studies of NP are harmonized,” Rikken et al wrote.¹ “A substantiated choice of the best scoring system and a meaningful cutoff value are essential prerequisites in this context.”

References

1. ECC Rikken, TW van Hal, JMPA van den Reek, et al. “What Is the Diagnostic Capacity of Existing Severity Scoring Tools for Nail Psoriasis?,” International Journal of Dermatology (2025): 1–7, https://doi.org/10.1111/ijd.70024.

2. TW van Hal, JM van den Reek, HM Groenewoud, et al. “Discovery of Arthritis in Psoriasis Patients for Early Rheumatological Referral (DAPPER): Protocol for a Longitudinal Observational Study,” JMIR Research Protocols 10, no. 11 (2021): e31647.

3. KM Klaassen, PC van de Kerkhof, MC Pasch, et al. “Scoring Nail Psoriasis,” Journal of the American Academy of Dermatology 70, no. 6 (2014): 1061–1066.