Assessment of functional somatic disorders in epidemiological research: Self-report questionnaires versus diagnostic...

Self-reported symptom questionnaires are often used for identifying individuals with functional somatic disorders (FSD) in epidemiological research. Studies on their validity in establishing clinically valid cases are, however, lacking. We aimed to compare and dissect the processes of identifying participants with FSD with symptom questionnaires and FSD diagnoses established by diagnostic interviews.

Individuals from the adult Danish population (n = 1590) filled in symptom questionnaires and participated in a diagnostic research interview, performed over telephone by trained family physicians. The two methods were described and compared in different steps: 1) Agreement on presence of symptoms, 2) agreement after FSD symptom pattern criteria had been applied, and 3) agreement on final FSD diagnoses.

Agreement on symptom presence was high (>82%). Using FSD symptom pattern criteria, the two methods agreed in 30-62% of cases within each category. Discrepancies were mainly due to participants fulfilling symptom patterns in the interview but not in the questionnaires. Agreement between final FSD questionnaire cases and final FSD interview diagnoses was moderate (>68%) with lower FSD prevalence in the interview (26.2% vs 44.5%). Discrepancies were largely explained by the interviewers assessing the symptom patterns to be caused by an alternative physical or mental condition.

Prevalence of final FSD diagnoses were markedly lower in the diagnostic interview compared to self-reported questionnaires cases; mainly because of the clinical evaluation of symptom attribution and impairment. Symptom questionnaires may be valuable as screening tools and as trans-diagnostic comparison while diagnostic interviews are necessary in establishing clinically significant FSD diagnoses.