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Measurement properties of the Polymyalgia Rheumatica Activity Score (PMR-AS): reliability, measurement error and
Noortje E S E P Kooijman1,2, David F Ten Cate1, Pauline M E T Bovens1
1Department of Rheumatology, Sint Maartenskliniek, Nijmegen, The Netherlands.
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 <10 and ≥10) was evaluated using Cohen's kappa (ᴋ).
Results:
A total of 124 patients contributed to the inter-rater analyses and 38 for test-retest analyses. For inter-rater reliability, the PMR-AS showed excellent agreement (ICC 0.93, 95% CI 0.90-0.95). Test-retest reliability was lower (ICC 0.66, 95% CI 0.38-0.82), reflecting lack of variability in disease activity (median PMR-AS 1.6). Measurement error was notable (inter-rater: SEM 2.46, SDD 6.83; test-retest: SEM 2.07, SDC 5.75), but discriminative validity was good (AUC 0.93, ᴋ 0.69).
Conclusion:
The PMR-AS demonstrated excellent inter-rater reliability and discriminative validity, supporting its use for classification of disease activity. However, its measurement error may limit interpretation of small intra-individual changes.
Insights
The Polymyalgia Rheumatica Activity Score (PMR-AS) shows excellent reliability between raters and good discriminative validity for assessing disease activity in Polymyalgia Rheumatica (PMR). However, its measurement error may affect interpretation of minor changes in individual patient scores.
Area of Science:
- Rheumatology
- Clinical Measurement
- Disease Activity Assessment
Background:
- The Polymyalgia Rheumatica Activity Score (PMR-AS) is a validated tool for measuring disease activity in Polymyalgia Rheumatica (PMR).
- Previous studies have not fully evaluated the reliability, measurement error, and discriminative validity of the PMR-AS.
Purpose of the Study:
- To assess the inter-rater reliability, test-retest reliability, measurement error, and discriminative validity of the PMR-AS.
- To determine the clinical utility of the PMR-AS in routine care and clinical trials for Polymyalgia Rheumatica.
Main Methods:
- Cross-sectional cohort study of 124 PMR patients for inter-rater reliability and measurement error.
- Analysis of data from 38 PMR patients in randomized trials for test-retest reliability and measurement error.
- Statistical analyses included intraclass correlation coefficients (ICC), standard error of measurement (SEM), smallest detectable change (SDC), and ROC analyses.
Main Results:
- Excellent inter-rater reliability (ICC 0.93) and good discriminative validity (AUC 0.93) were observed for the PMR-AS.
- Test-retest reliability was moderate (ICC 0.66), with notable measurement error (inter-rater SDC 6.83; test-retest SDC 5.75).
- Agreement between disease activity categories (PMR-AS <10 vs. ≥10) was substantial (Cohen's kappa 0.69).
Conclusions:
- The PMR-AS demonstrates strong inter-rater reliability and discriminative validity, supporting its use in classifying Polymyalgia Rheumatica disease activity.
- The identified measurement error suggests caution when interpreting small changes in individual patient scores over time.
- Further validation is recommended to refine the clinical application of the PMR-AS in Polymyalgia Rheumatica management.
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