Validity and diagnostic overlap of functional somatic syndrome diagnoses.

Objective: We present the first study that investigates the validity and the diagnostic overlap of the three main functional somatic syndrome (FSS) diagnoses, i.e. chronic fatigue syndrome (CFS), fibromyalgia (FM), and irritable bowel syndrome (IBS), irrespective of help-seeking behaviour or diagnostic habits, and irrespective of differences in diagnostic thresholds for chronicity or symptom interference.