Date sent: Sun, 10 Oct 1999 U.S. Case Definition of Chronic Fatigue Syndrome: Diagnostic and Theoretical Issues [ Journal of Chronic Fatigue Syndrome (The Haworth Medical Press, an imprint of The Haworth Press, Inc.) Vol. 5, No. 3/4, 1999 ] Leonard A. Jason Caroline P. King Judith A. Richman Renee R. Taylor Susan R. Torres Sharon Song SUMMARY. In 1994, researchers from the U.S. Centers for Disease Control and Prevention developed a revised case definition of chronic fatigue syndrome (CFS) (1), a complex illness characterized by debilitating fatigue and a number of accompanying flu-like symptoms. Although Fukuda and associates intended to resolve complexities surrounding the classification of individuals with CFS stemming from previous definitional criteria (1), significant problems with the revised criteria endure. This article highlights reliability issues and other conceptual and operational difficulties inherent in the current U.S. definition of CFS (1). We employ case studies derived from a community-based epidemiological study of chronic fatigue syndrome (2) to illustrate examples of the potential for misclassification of individuals with CFS using the current U.S. criteria (1). Moreover, we suggest alternative approaches to classification and ways to operationalize specific concepts embedded in the current U.S. criteria (1). KEYWORDS. Epidemiology, fatigue Historically, many chronic illnesses have been difficult to define, particularly when the exact causal agents of the illness are not known, physical signs and symptoms are nonspecific or variable, and diagnostic laboratory tests are not applicable, unavailable, or have poor specificity and sensitivity (3). With chronic illnesses of this nature, standard case definitions have had to be developed through the consensus of expert committees (e.g., carpal tunnel syndrome, rheumatoid arthritis, systemic lupus erythematosus, and various psychiatric disorders) (3,4). Chronic fatigue syndrome (CFS) is one of these difficult illnesses to define, and despite years of research, it remains a poorly understood and controversial syndrome (5). Even the classification of CFS has been controversial, and this might reflect the fact that some researchers consider CFS a medical-neurological illness, whereas others consider it a psychiatric disorder (6). One factor that has confounded research on CFS is the lack of consensus among health care professionals regarding the interpretation and application of the diagnostic criteria for CFS. Criteria need to possess adequate sensitivity, identifying all those with CFS, and adequate specificity, distinguishing those who have other medical or immunological disorders from those who have CFS. Attempts to specify the diagnostic criteria for this syndrome have sparked considerable debate and controversy (6,7). Since its emergence as a new disease category in the 1980s, four definitions of CFS have been proposed, but none have been derived empirically (8). Perhaps as a result, clinicians and health care professionals working with chronically fatigued clients have noted a number of difficulties with each of these case definitions. One study that compared the original U.S. case definition (9) and the British and Australian case definitions (10,11) found major differences in criteria between each of the three case definitions. Tiersky and associates (12) have produced data indicating that the 1988 and 1994 U.S. CFS case definition criteria (9,1) identify distinct patient groups. These differences substantially affected the number of patients meeting the criteria of each definition (13). Subtle changes in the wording, interpretation, and application of the diagnostic criteria used to identify people with CFS can critically influence prevalence rates and change the characteristics of the samples being defined. Several recent CFS community-based prevalence studies using the current U.S. criteria (1) produced prevalence estimates that differ by a factor of 10, thus suggesting that these investigators might be identifying different samples of people with CFS (14-16). Unclear criteria might be responsible for these discrepant rates. The present paper will focus specifically on the practical and theoretical issues in using the current U.S. case definition (1), as it is the most commonly used criteria for diagnosing CFS. [ Article copies available for a fee from The Haworth Document Delivery Service: 1-800-342-9678. E-mail address: getinfo@haworthpressinc.com .] Leonard A. Jason and Caroline P. King are affiliated with DePaul University. Judith A. Richman is affiliated with the University of Illinois at Chicago. Renee R. Taylor, Susan R. Torres and Sharon Song are affiliated with DePaul University. Address correspondence to: Leonard Jason, PhD, Department of Psychology, DePaul University, 2219 N. Kenmore Avenue, Chicago, IL 60614. The authors appreciate the financial support provided by NIAD grant # A136295. The authors would like to express their appreciation to Erin Frankenberry, Susan Klein, and Guy Fricano for their constructive advice.