• Alessandro Chiarotto, Leo D Roorda, Martine H Crins, Maarten Boers, Raymond W Ostelo, and Caroline B Terwee.
• Department of Health Sciences, Faculty of Science, Amsterdam Movement Sciences Research Institute, VU University, Amsterdam, The Netherlands; Department of General Practice, Erasmus MC, University Medical Center, Rotterdam, The Netherlands. Electronic address: a.chiarotto@vu.nl.
• Arch Phys Med Rehabil. 2020 Feb 1; 101 (2): 297-308.

ObjectiveTo compare dimensionality, item-level characteristics, scale-level reliability, and construct validity of PROMIS Physical Function short forms (PROMIS-PF) and 24-item Roland Morris Disability Questionnaire (RMDQ-24) in patients with chronic low back pain (LBP).DesignCross-sectional study.SettingSecondary care center for rehabilitation and rheumatology.ParticipantsPatients with nonspecific LBP ≥3 months (N=768). Mean age was 49±13 years, 77% were female, and 54% displayed pain for more than 5 years.InterventionsNot applicable.Main Outcome MeasuresDutch versions of the 4-, 6-, 8-, 10-, and 20-item PROMIS-PF and of the RMDQ-24.ResultsPROMIS-PF-6, PROMIS-PF-8, and RMDQ-24 exhibited sufficient unidimensionality (confirmatory factor analysis: comparative fit index>0.950, Tucker-Lewis index>0.950, root means square error of approximation<0.060), whereas the other instruments did not. All instruments were free of local dependence except PROMIS-PF-20 with 4 item pairs with clear residual correlations. Mokken scale analysis found 1 nonmonotone item for PROMIS-PF-20 and 8 for RMDQ-24 (ie, the probability of endorsing these items was not increasing with increasing level on the underlying construct). PROMIS-PF-20 displayed 2 misfitting items (S-χ2P value>.001). Two-parameter item response theory models found 2 items with low discrimination for RMDQ-24. All other instruments had adequate fit statistics and item parameters. PROMIS-PF-20 displayed the best scale-level reliability. Construct validity was sufficient for all instruments as all hypotheses on expected correlations with other instruments and differences between relevant subgroups were met.ConclusionsPROMIS-PF-6, PROMIS-PF-8, and RMDQ-24 exhibited better unidimensionality, whereas PROMIS-PF-4, PROMIS-PF-6, PROMIS-PF-8, and PROMIS-PF-10 showed superior item-level characteristics. PROMIS-PF-20 was the instrument with the best scale-level reliability. This study warrants assessment of other measurement properties of PROMIS-PF short forms in comparison with disease-specific physical functioning instruments in LBP.Copyright © 2019 American Congress of Rehabilitation Medicine. Published by Elsevier Inc. All rights reserved.

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