Response to Letter to the Editor: A misleading CFS prevalence estimate in DanFunD.

Abstract

In his Letter to the Editor, Michiel Tack [1] addresses a concern that our reported prevalence of chronic fatigue syndrome (CFS) is misleading, as it is higher than in many other publications. We understand this concern, as it relates to some of the fundamental problems within the discipline of functional somatic disorders (FSD). First, we have multiple consensusbased definitions for each syndrome, some of which are developed in highly skewed samples and not validated in general populations. Second, there is an urgent need for more valid (i.e. data-driven) phenotypic approaches that can be used as the gold standard for defining FSD. Third, in epidemiological research, we need broader symptom clusters that include both severely ill individuals but also those who have the syndrome in abbreviated or milder forms [2–5]. In light of these fundamental problems, DanFunD was initiated 10 years ago as a large-scale epidemiological study with the aim of unravelling the epidemiology of various FSD and diagnostic analogues [6]. In order to achieve this aim, we included standard criteria for five common syndromes, which: