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Defining Chronic Fatigue Syndrome: Methodological Challenges

Full Title: Defining Chronic Fatigue Syndrome: Methodological Challenges
Journal: Journal of Chronic Fatigue Syndrome, Vol. 7(3) 2000, pp. 17-32
Authors: Leonard A. Jason, PhD; Caroline P. King, MA; Renee R. Taylor, PhD; Cara Kennedy, BA
Affiliation: DePaul University.
Address correspondence to: Leonard A. Jason, PhD, Department of Psychology, DePaul University, 2219 North Kenmore Avenue, Chicago, IL 60614 (E-mail: ).
Financial support for this study was provided by NIAID Grant Number A136295.


Accurate diagnosis of Chronic Fatigue Syndrome (CFS) is greatly complicated by the vague wording of many of the major diagnostic criteria (i.e., substantial reductions in previous levels of occupational, educational, social, or personal activities) and the absence of guidelines for health care professionals to follow.

The lack of operationally explicit criteria has forced health care professionals to rely heavily on their own clinical judgement, which may be biased by personal and highly idiosyncratic factors. Thus, in the case of CFS, the lack of consensus among clinicians regarding the interpretation and application of the diagnostic criteria has likely produced problems in diagnostic reliability.

Data from a recent community based epidemiologic study are presented to illustrate these problems and provide recommendations for improving criterion reliability.

KEYWORDS. Chronic Fatigue Syndrome, case definition, diagnosis


Chronic Fatigue Syndrome (CFS) is an illness that results in severe, prolonged fatigue as well as neuropsychiatric, rheumatological, and infectious symptoms (1). Despite several years of research, CFS remains a poorly understood and controversial disease (2). 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).

Some argue that syndromes such as CFS, Fibromyalgia and Irritable Bowel Syndrome may be better understood in terms of a unitary model of functional somatic distress, rather than as separate diagnostic entities (4). A recent study by Taylor, Jason and Schoeny (5) evaluated the diagnostic validity of conditions that have been labeled functional somatic syndromes. Latent variable models of functional somatic distress were estimated from the responses of 213 community members to a medical questionnaire. Results of confirmatory factor analysis supported diagnostic distinctions between five syndromes (fibromyalgia, chronic fatigue syndrome, somatic depression, somatic anxiety, and irritable bowel syndrome). The diagnostic validity of the latent constructs of FMS and CFS emerging from this five-factor model were cross-validated using findings from an independent physician evaluation.

The primary goal of classifying any disease or illness is to group together patients who have an illness that may have many manifestations, but a common underlying pathophysiological pathway (6). Historically, classification of such patients has facilitated research on the etiology and treatment of illnesses and diseases before the underlying pathophysiological pathway was identified by simply classifying these illnesses as syndromes of signs and symptoms (e.g., systemic lupus erythematosus or tuberculosis). It has been suggested that a similar classification process may help facilitate research in the field of CFS (6). At present, a variety of classification methods have been used to classify fatigue. Researchers from the Centers for Disease Control, such as Fukuda and associates (7), have used clinical approaches to the classification of fatigue whereas researchers such as Haley, Kurt, and Horn (8) and Hall, Sanders, and Repologle (9) have used statistical approaches. Methods to confirm the current U.S. CFS case definition have also been established by Komaroff, Fagioli, Geiger et al. (10) and Nisenbaum, Reyes, Mawle, and Reeves (11).

In the absence of laboratory tests or other objective indicators, case identification of CFS depends primarily on information obtained through clinical interviews (12). The reliability of the clinical interview with patients with CFS is therefore crucial. However, one factor that has continued to complicate progress toward understanding CFS is the lack of consensus among health care professionals regarding the interpretation and application of the diagnostic criteria for CFS (13).

Studies examining sources of diagnostic unreliability have shown that subject, occasion, and information variance account for only a small portion of diagnostic reliability (14). However, criterion variance, differences in the formal inclusion and exclusion criteria used by clinicians to classify patients’ data into diagnostic categories, accounts for the largest source of diagnostic unreliability. Therefore, improvement in diagnostic reliability is primarily dependent on reducing criterion variance as a source of unreliability. Criterion variance is most likely to occur when operationally explicit criteria do not exist for diagnostic categories (15).

The current U.S. case definition for CFS (7) is characterized by vaguely worded criteria that lack operational definitions and guidelines to assist health care professionals in their interpretation and application of the diagnostic criteria (13). In addition, because of the lack of objective clinical or laboratory diagnostic markers, the current case definition for CFS is dependent upon subjective factors and patient self-report (7). For example, symptoms of sore throat or lymph node pain can only be infrequently confirmed by a physician, and the other symptomatic criteria are even more difficult to confirm because of their ambiguous nature (e.g., difficulty concentrating) (3). Together, this combination of poorly defined diagnostic criteria and lack of objective clinical diagnostic markers has made the assessment and diagnosis of CFS a complex and difficult process. As a result, health care professionals have been forced to rely on their own clinical judgement, which may be biased by personal and highly idiosyncratic factors. Efforts to develop objective clinical means of distinguishing patients with CFS from those with other illnesses are therefore greatly needed. More specifically, precise criteria for identifying key symptoms of CFS are essential to reduce criterion variance and obtain adequate diagnostic reliability.

An important criterion for the current case definition of CFS (7) involves the need to have a substantial reduction in critical areas of functioning. Therefore, there is a need to assess reductions in activity levels in the realms of social, occupational, and familial functioning. One instrument that provides standardized scales for assessing physical functioning, physical-role functioning, emotional-role functioning, and social functioning is the SF-36 Health Survey (16). The SF-36 Health Survey is a standardized questionnaire comprised of eight multi-item scales measuring eight different aspects of health. Use of these functioning scales with cutoff scores would likely improve clinicians’ ability to accurately identify individuals with limited capacities for functioning and assess reductions in social, occupational, and familial functioning. Research (17) has demonstrated the SF-36 to be a useful and reliable instrument for assessing functional status in patients with CFS and for distinguishing patients with CFS from patients with other fatiguing conditions on the basis of functional status. In Buchwald et al.‘s (17) study, individuals with CFS had mean scores of 40 or less for the following scales: physical functioning, general health, vitality, body pain, physical-role function, and social function. One study (18) found that patients with CFS had mean scores below 40 on the following scales: physical role-function, vitality and general health. A treatment study (19) used the physical functioning scale, and patients with CFS mean baseline scores were below 40.

The current CFS case definition also requires the occurrence of four or more minor symptoms (7). A number of problems with the definitions of the minor criteria (i.e., difficulties with memory and concentration; sore throat; lymph node pain; muscle pain; joint pain; headache; unrefreshing sleep; and post-exertional malaise) have resulted from the lack of operational definitions and the absence of guidelines for clinicians to follow when assessing patients presenting with chronic fatigue. Unfortunately, many of the minor symptoms for CFS are subjective in nature and the assessment of subjective symptoms is inherently problematic as these symptoms often cannot be directly observed or objectively measured. A critical issue concerning the assessment of the minor symptom criteria is the use of binary classification (i.e., occurrence versus nonoccurrence). Many patients with CFS have symptoms for extended periods of time, but because of imprecise wording of the binary symptoms, some individuals with CFS are not scored as having a symptom when they actually have it.

Three of the most commonly used methods to assess symptoms are the visual analog scale (VAS), the verbal rating scale (VRS), and the numerical rating scale (NRS) (20). The primary advantages of using numeric rating scales (NRS) are the following: they are easy to administer and score, they have demonstrated good evidence of construct validity, compliance with the measurement task is high, and the data can be treated as ratio data (20). CFS symptoms could easily be rated on 100 point rating scales rather than binary scales involving occurrence vs. non-occurrence.

Another problem with the minor symptom criteria in terms of diagnostic reliability is the specification that the four or more minor symptoms must not predate the fatigue (7). Clarification is needed as to what exactly “predating” the fatigue means. If a patient had only four minor symptoms and one of the symptoms (e.g., sore throat) began three months before the overwhelming fatigue, the definition suggests that this individual would be excluded from the diagnosis of CFS. Some individuals who develop CFS experience a prodromal phase, in which symptoms begin to appear in the year prior to the onset of fatigue. It is therefore unclear why such symptoms would not be counted as minor symptoms. As a result, some clinicians might allow symptoms from the prodromal phase to count toward fulfilling the diagnostic criteria while others might not. This lack of precision in the definition is likely to lead to problems in interrater reliability, as well as the misdiagnosis of patients who do and do not have CFS.

Given the absence of specific thresholds and scoring rules for the symptomatic criteria of the current U.S. CFS case definition, the probability of criterion variance is greatly increased. More importantly, the absence of specific thresholds and scoring rules has likely compromised the diagnostic reliability of the current U.S. CFS case definition. The present study uses data from a CFS epidemiologic study to consider parameters of scoring rules that might be developed for different criteria specified by the U.S. case definition.

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