Beyond a symptom count: addressing the difficulties of measuring Functional Somatic Symptoms

Functional Somatic Symptoms (FSS) are a constellation of symptoms such as pain, fatigue, and dizziness, which are not explained by objectively measurable underlying pathology (also called medically unexplained). These symptoms are very heterogeneous, and therefore, difficult to assess. The overall aim of this thesis was to address three methodological issues in the assessment of FSS, namely: 1) The heterogeneity of FSS and the problems of using sum-scores, 2) the uncertainty concerning which symptoms are most relevant for FSS assessment, and 3) the ambiguity of the meaning of sum-scores for different subgroups. To address these issues, sophisticated psychometric methods from Item Response Theory were applied to three FSS questionnaires. The findings suggest that items related to weakness, heaviness in extremities, and nausea consistently provide accurate measurements about FSS severity. Moreover, it was found that some symptoms could be biased by sex, which means that these items may be more accurate at measuring FSS severity in either males or females. Joint pain was found not to be very informative when measuring FSS in older adults. Finally, we found that participants did not tend to cluster in subgroups based on their symptoms (e.g., gastrointestinal or musculoskeletal), but based on their overall severity of symptoms. This means that the severity of FSS symptoms could be more important for identifying and classifying individuals than the combination of symptoms. These results can contribute to the improvement of measurement tools for assessing FSS.