1 The adoption of Chronic Fatigue Syndrome/Myalgic

Purpose: Prevalence estimates have been based on several case definitions of Chronic Fatigue Syndrome (CFS). The purpose of this paper is to provide a rigorous overview of their application in prevalence research. Methods: A systematic review of primary studies reporting the prevalence of CFS since 1990 was conducted. Studies were summarized according to study design, prevalence estimates, and case definition used to ascertain cases. Results: Thirty one studies were retrieved and eight different case definitions were found. Early estimates of CFS prevalence were based on the 1988 CDC [1], Australian [2], and Oxford [3]. The 1994 Centers for Disease Control and Prevention (CDC) [4] however, has been adopted internationally, as a general standard. Only one study has reported prevalence according to the more recent, Canadian Consensus Criteria (CCC) [5]. Additional estimates were also found according to definitions by Ho-Yen [6], the 2005 CDC empirical definition [7], and an epidemiological case definition (ECD) [8]. Conclusion: Advances in clinical case definitions during the past 10 years such as the CCC [5] ha