The impact of sex and gender on Fibromyalgia Syndrome: data from the Italian Fibromyalgia Registry

Most fibromyalgia (FM) patients are women, with a female-to-male ratio of 3:1. While sex differences in pain perception and description have been reported, the impact of sex and gender on FM severity remains controversial. Additionally, sex-related differential item functioning (DIF) in FM assessment tools has not yet been explored. The primary aim of this study was to analyze sex- and gender-related differences in FM severity using data from a web-based FM registry. The secondary aim was to assess sex-related DIF in three commonly used questionnaires: the Polysymptomatic Distress Scale (PDS), the modified Fibromyalgia Assessment Status (ModFAS), and the revised Fibromyalgia Impact Questionnaire (FIQR). Data from 331 male and 331 female patients, matched for age and body mass index (BMI) and fulfilling ACR 2010/2011 criteria, were retrospectively collected from the Italian Fibromyalgia Registry. Multivariate analyses were conducted on the overall and sex-stratified populations. Sex-related DIF was assessed using a hybrid Ordinal Logistic Regression/Item Response Theory method. Female sex was significantly associated with greater physical impairment, despite no differences in overall disease severity. In stratified analyses, married status influenced disease impact and burden in women, whereas BMI was associated with higher disability in men. Significant sex-related DIF was detected in one item of the Symptom Severity Scale of the PDS. Women with FM experience greater physical impairment than men, despite similar disease severity. Sociodemographic factors influence FM differently across sexes. Despite minor DIF, the three FM-specific questionnaires appear valid for use in both male and female patients.