Construct Validity of the Modified Numeric Rating Scale of Patient Global Assessment in Psoriatic Arthritis

Abstract

Objective.The construct validity of the patient global health assessment (PGA) in psoriatic arthritis (PsA) has not been analyzed, despite its common use. We evaluated the construct validity of a numeric rating scale (NRS) of the PGA in PsA.Methods.Patients with PsA who fulfilled the ClASsification for Psoriatic ARthritis (CASPAR) criteria were recruited at a tertiary referral center. Demographic data were collected and PGA data were determined from administration of an 11-point NRS (0 to 10 points representing best to worst status). Convergent and discriminant validity were evaluated by correlation between PGA and clinical variables. Patients were grouped as having severe disease based on Disease Activity Score 28-joint count (DAS28) > 5.1, Health Assessment Questionnaire (HAQ) > 1.0, walking with aids, and social welfare-dependent. Patients were grouped as being in remission by DAS28 < 2.6 and the Minimal Disease Activity Criteria. Known-group validity of PGA was evaluated.Results.A total of 125 patients (52% men) were studied. Convergent validity revealed strong correlations of PGA with pain score, HAQ, and DAS28; and weak correlations with skin severity score, physician’s global assessment and morning stiffness. In multivariate analysis, PGA was associated with pain, physical function, mental function, and skin severity score. PGA distinguished different levels of severity well, as determined by comparison with different known groups with large effect sizes.Conclusion.Judged on an NRS, the PGA had good construct validity and satisfactorily distinguished all levels of severity in PsA.