Development and validation of the Physician's Global Assessment of Fingernail Psoriasis

Background: Several clinician-rated scoring systems are available to assess nail psoriasis severity, but only one has been partially validated.

Objective: To develop and validate the Physician's Global Assessment of Fingernail Psoriasis (PGA-F), a new clinician-rated severity scale.

Methods: A literature review, concept elicitation, pilot cognitive debriefing and clinical expert consultations informed the development of the PGA-F. A multistage mixed-methods analysis consisted of practising dermatologist cognitive interviews (n = 10) for instrument clarity, relevance and comprehensiveness. Inter-rater reliability (IRR) of ratings from dermatologists (n = 22) and clinical trial investigators (n = 8) was tested using many-facet Rasch analysis. Concurrent validity between the PGA-F and modified Nail Psoriasis Severity Index (mNAPSI) at screening and baseline was assessed along with the degree of discrimination. Intraclass correlation coefficient (ICC) for single raters at multiple assessments determined IRR.

Results: The PGA-F synthesizes severity ratings across multiple disease features that classify individuals into 1 of 5 levels (clear to severe). Cognitive interviews confirmed content validity: all (n = 10, 100%) participants who agreed clinical criteria were consistent with nail psoriasis; no mismatched severity levels; and training photographs were realistic representations. All PGA-F items were locally independent and targeted patients along the severity continuum with complementary precision (item fit statistics: < the 1.5 acceptability threshold; exact agreements among the dermatologists [44%] and trial investigators [61.5%] exceeded 40% of acceptability threshold). Clinician reliability exceeded the threshold of acceptability for dermatologists and clinical trial investigators: 0.85 and 0.73, respectively. There was adequate correlation (>0.30) between mNAPSI and PGA-F at baseline and Week 26 with significant discrimination of severity and monotonic increases in the mNAPSI for each level of categorical severity on the PGA-F. ICC results for each type of IRR indicate that clinicians were consistent in individual patient ratings.

Conclusion: The PGA-F is a rapid, valid and reliable clinician-rated severity scale for use in clinical practice and research.