Purpose: Efficient management of fibromyalgia (FM) requires precise measurement of FM-specific symptoms. Our objective was to assess the measurement properties of the Patient-Reported Outcome Measurement Information System (PROMIS) fatigue item bank (FIB) in people with FM. Methods: We applied classical psychometric and item response theory methods to cross-sectional PROMIS-FIB data from two samples. Data on the clinical FM sample were obtained at a tertiary medical center. Data for the U.S. general population sample were obtained from the PROMIS network. The full 95-item bank was administered to both samples. We investigated dimensionality of the item bank in both samples by separately fitting a bifactor model with two group factors; experience and impact. We assessed measurement invariance between samples, and we explored an alternate factor structure with the normative sample and subsequently confirmed that structure in the clinical sample. Finally, we assessed whether reporting FM subdomain scores added value over reporting a single total score. Results: The item bank was dominated by a general fatigue factor. The fit of the initial bifactor model and evidence of measurement invariance indicated that the same constructs were measured across the samples. An alternative bifactor model with three group factors demonstrated slightly improved fit. Subdomain scores add value over a total score. Conclusions: We demonstrated that the PROMIS-FIB is appropriate for measuring fatigue in clinical samples of FM patients. The construct can be presented by a single score; however, subdomain scores for the three group factors identified in the alternative model may also be reported.
Bibliographical noteFunding Information:
This study was supported in part by the Center for Clinical and Translational Science (CCaTS) at Mayo Clinic. Mayo Clinic's CCaTS is funded by the National Institute of Health's (NIH) Clinical and Translational Science Awards (CTSA) program, Grant Number UL1TR000135, from the National Center for Advancing Translational Sciences (NCATS). Its contents are solely the responsibility of the authors and do not necessarily represent the official view of CCaTS, NCATS, or NIH. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Additional support was provided by the Mayo Clinic Robert D. and Patricia E. Kern Center for the Science of Health Care Delivery (Yost, Lee) and the Mayo Clinic Department of Health Sciences Research (Yost, Waller).
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- Item response theory
- Patient-Reported Outcome Measurement Information System (PROMIS)