Measurement properties of the Polymyalgia Rheumatica Activity Score (PMR-AS): reliability, measurement error and discriminative validity

OBJECTIVES: The Polymyalgia Rheumatica Activity Score (PMR-AS) is the best validated composite disease activity measure in PMR to date, but its reliability, measurement error and discriminative validity have not been studied. METHODS: Inter-rater reliability, measurement error and discriminative validity were assessed in a cross-sectional cohort of PMR patients in routine care (April-August 2025). For inter-rater reliability and measurement error, five physicians scored the PMR-AS, with two independently evaluating each patient at the same visit. Test-retest reliability and measurement error were assessed using data from two randomized trials in newly diagnosed PMR (2019-2024, EULAR/ACR criteria positive). Patients with two visits 2-4 weeks apart and no reported change in disease state were included. Analyses included intraclass correlation coefficients (ICCs), standard error of measurement (SEM), smallest detectable change (SDC/SDD) and ROC analyses. Agreement between disease activity categories (PMR-AS