Functional Somatic Symptoms (FSS) are physical symptoms that cannot be attributed to underlying pathology. Their severity is often measured with sum scores on questionnaires; however, this may not adequately reflect FSS severity in subgroups of patients. We aimed to identify the items of the somatization section of the Composite International Diagnostic Interview that best discriminate FSS severity levels, and to assess their functioning in sex and age subgroups. We applied the two-parameter logistic model to 19 items in a population-representative cohort of 962 participants. Subsequently, we examined differential item functioning (DIF). “Localized (muscle) weakness” was the most discriminative item of FSS severity. “Abdominal pain” consistently showed DIF by sex, with males reporting it at higher FSS severity. There was no consistent DIF by age, however, “Joint pain” showed poor discrimination of FSS severity in older adults. These findings could be helpful for the development of better assessment instruments for FSS, which can improve both future research and clinical care.

功能性躯体症状 (FSS) 是不能归因于潜在病理的身体症状。他们的严重程度通常用问卷上的总分来衡量；然而，这可能无法充分反映患者亚组的 FSS 严重程度。我们旨在确定综合国际诊断访谈中躯体化部分最能区分 FSS 严重程度的项目，并评估它们在性别和年龄亚组中的功能。我们将两参数逻辑模型应用于 962 名参与者的人口代表性队列中的 19 个项目。随后，我们检查了差异项目功能（DIF）。“局部（肌肉）无力”是 FSS 严重程度最具区别性的项目。“腹痛”始终按性别显示 DIF，男性报告其 FSS 严重程度较高。年龄没有一致的 DIF，然而，“关节痛”表明老年人对 FSS 严重程度的辨别能力较差。这些发现可能有助于开发更好的 FSS 评估工具，从而改善未来的研究和临床护理。