Background: The prevalence of anxiety is three times greater in patients with chronic diseases than in the general population. There has recently been growing interest in the roles of inflammation in contributing to the development of anxiety in people with Immune-mediated inflammatory diseases (IMID). Patient-reported outcome measures can facilitate the assessment of physical and psychological functioning. In recent years various patient outcome measures (PROMs) have been implemented. The National Institutes of Health (NIH)'s Patient-Reported Outcomes Measurement Information System (PROMIS®) is a set of publicly available and standardized Patient-Reported Outcomes (PROs) that cover physical appearance, mental health, and social health. The PROMIS® has been built through an Item Response Theory approach (IRT), a model-based measurement in which trait level estimates depend on both persons' responses and on the properties of the items that were administered. The aim of this study is to test the psychometric properties of the original anxiety items of the PROMIS® in a cohort of outpatients with IMIDs.Methods: We translated the original eight anxiety items from the PROMIS into Italian and administered them to consecutive outpatients affected by Inflammatory Bowel disease (n = 246), rheumatological (n = 100) and dermatological (n = 43) diseases, and healthy volunteers (n = 280). We analyzed the data through an Item Response Theory (IRT) analysis to evaluate the psychometric properties of the Italian adaptation of the PROMIS® anxiety short form.Results: Taken together, Confirmatory Factor Analysis and Exploratory Factor analysis suggested that the unidimensionality assumption if the instrument holds. The instrument has excellent reliability from a Classical Theory of Test (CTT) standpoint (Cronbach's ⍺ = .94, McDonald's ω = .95). The 1PL Graded Response Model (GRM) model provided a good balance between the goodness of fit and parsimony (BIC = 9099.82) as compared to the 2PL GRM model (BIC = 9111.02). We did not find signs of Differential Item Functioning (DIF), but we found that the local independence assumption was violated for some items. The analysis of the test reliability curve suggested that the instrument is most reliable for higher levels of the latent trait of anxiety.Discussion: The Italian adaptation of the PROMIS® anxiety short form shows acceptable psychometric properties both from a CTT and an IRT standpoint. Its compatibility with the 1 parameter logistic (1PL) model implies that its total score is a sufficient statistic for the latent anxiety trait. The TRC shows that this instrument is mostly informative for people with higher levels of anxiety, making it particularly suitable for clinical populations such as IMID patients.
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