Case Definition Of Chronic Fatigue Syndrome Research Articles

Pain in adolescent chronic fatigue following Epstein-Barr virus infection.

Objectives Acute Epstein-Barr virus (EBV) infection is a trigger of Chronic Fatigue (CF) and Chronic Fatigue Syndrome (CFS). The aim of this cross-sectional study was to investigate pain symptoms and pressure pain thresholds in fatigued and non-fatigued adolescents six months after acute EBV-infection, and in healthy controls. This study is part of the CEBA-project (CF following acute EBV infection in adolescents). Methods A total of 195 adolescents (12-20 years old) that had undergone an acute EBV infection six months prior to assessment were divided into fatigued (EBV CF+) and non-fatigued (EBV CF-) cases based on questionnaire score. The EBV CF+ cases were further sub-divided according to case definitions of CFS. In addition, a group of seventy healthy controls was included. Symptoms were mapped with questionnaires. Pressure pain thresholds were measured through pressure algometry. One way ANOVA were used for between-group analyses. Linear regression analyses were used to explore associations between Pediatric Quality of Life (dependent variable), pain symptoms and other variables within the EBV (CF+) group. Results The EBV CF+ group had significantly higher scores for pain symptoms as compared with the EBV CF- group and healthy controls, but pressure pain threshold did not differ significantly. The number of pain symptoms as well as pain severity were strongly and independently associated with quality of life. Conclusions CF and CFS following acute EBV-infection in adolescents is characterized by high pain symptom burden, which in turn is associated with a decline in quality of life. Pain in CF and CFS is of considerable clinical importance, and should be a focal point for further investigation and intervention in these patient groups.

The DePaul Symptom Questionnaire-2: a validation study

ABSTRACT Background: The DePaul Symptom Questionnaire (DSQ) was developed to assess the symptomatology and case definition fulfillment of individuals with myalgic encephalomyelitis (ME) and chronic fatigue syndrome (CFS). The questionnaire was recently revised to improve its psychometric properties, increase its diagnostic reliability, and assess symptoms required by case definitions. The resulting instrument was named the DSQ-2. Purpose: The current study sought to evaluate the utility and reliability of the new and revised items in the DSQ-2. Method: A cross-sectional sample of 399 adults with ME or CFS was recruited to complete the DSQ-2. Results: Descriptive analyses of the DSQ-2 suggest that the new and revised items enhance the instrument’s ability to assess certain symptom domains and evaluate recent case definitions. Additionally, an exploratory factor analysis resulted in an eight-factor solution: post-exertional malaise, cognitive impairment, fever and flu, pain, sleep disruption, orthostatic intolerance, genitourinary issues, and temperature intolerance. The items within each factor demonstrated strong internal consistency reliability (Cronbach’s alphas = .73 – .91). Conclusion: These analyses indicate that the DSQ-2 offers a more thorough and precise understanding ME and CFS symptomology and case definition fulfillment.

Clinical symptoms and markers of disease mechanisms in adolescent chronic fatigue following Epstein-Barr virus infection: An exploratory cross-sectional study

IntroductionAcute Epstein-Barr virus (EBV) infection is a trigger of chronic fatigue (CF) and Chronic Fatigue Syndrome (CFS). The aim of this cross-sectional study was to explore clinical symptoms as well as markers of disease mechanisms in fatigued and non-fatigued adolescents 6 months after EBV-infection, and in healthy controls. Materials and methodsA total of 200 adolescents (12–20 years old) with acute EBV infection were assessed 6 months after the initial infectious event and divided into fatigued (EBV CF+) and non-fatigued (EBV CF−) cases based on questionnaire score. The EBV CF+ cases were further sub-divided according to case definitions of CFS. In addition, a group of 70 healthy controls with similar distribution of sex and age was included. Symptoms were mapped with a questionnaire. Laboratory assays included EBV PCR and serology; detailed blood leukocyte phenotyping and serum high-sensitive C-reactive protein; and plasma and urine cortisol and catecholamines. Assessment of autonomic activity was performed with continuous, non-invasive monitoring of cardiovascular variables during supine rest, controlled breathing and upright standing. Differences between EBV CF+ and EBV CF− were assessed by simple and multiple linear regression adjusting for sex as well as symptoms of depression and anxiety. A p-value ≤ 0.05 was considered statistically significant. This study is part of the CEBA-project (Chronic fatigue following acute Epstein-Barr virus infection in adolescents). ResultsThe EBV CF+ group had significantly higher scores for all clinical symptoms. All markers of infection and most immune, neuroendocrine and autonomic markers were similar across the EBV CF+ and EBV CF− group. However, the EBV CF+ group had slightly higher serum C-reactive protein (0.48 vs 0.43 mg/L, p = 0.031, high-sensitive assay), total T cell (CD3+) count (median 1573 vs 1481 × 106 cells/L, p = 0.012), plasma norepinephrine (1420 vs 1113 pmol/L, p = 0.01) and plasma epinephrine (363 vs 237 nmol/L, p = 0.032); lower low-frequency:high frequency (LF/HF) ratio of heart rate variability at supine rest (0.63 vs 0.76, p = 0.008); and an attenuated decline in LF/HF ratio during controlled breathing (−0.11 vs −0.25, p = 0.002). Subgrouping according to different CFS diagnostic criteria did not significantly alter the results. Within the EBV CF+ group, there were no strong correlations between clinical symptoms and markers of disease mechanisms. In a multiple regression analysis, serum CRP levels were independently associated with serum cortisol (B = 4.5 × 10−4, p < 0.001), urine norepinephrine (B = 9.6 × 10−2, p = 0.044) and high-frequency power of heart rate variability (B = –3.7 × 10−2, p = 0.024). ConclusionsIn adolescents, CF and CFS 6 months after acute EBV infection are associated with high symptom burden, but no signs of increased viral load and only subtle alterations of immune, autonomic, and neuroendocrine markers of which no one is strongly correlated with symptom scores. A slight sympathetic over parasympathetic predominance is evident in CF and might explain slightly increased CRP levels.

Myalgic encephalomyelitis and chronic fatigue syndrome case definitions: effects of requiring a substantial reduction in functioning

ABSTRACT Background: Current case definitions for myalgic encephalomyelitis (ME) and chronic fatigue syndrome (CFS) require an individual to report a ‘substantial reduction’ in activity levels, when compared to premorbid functioning. However, little guidance is provided on how to measure these reductions, as well as what level of reduction should be deemed ‘substantial,’ leading to inconsistencies in how this criterion is applied across research settings. Purpose: The current study examined the influence of substantial reduction criterion on case definitions. Method: The current study analyzed an international convenience sample of 1002 individuals with ME or CFS, 53 healthy controls, and 260 controls with other chronic illnesses. Results: Findings indicated that the utility of the substantial reduction criterion varied by case definition, with more stringent case definitions not needing this criterion to identify cases. Conclusion: These results suggest that the requirement of a substantial reduction in functioning may be redundant when case definitions specify that individuals must endorse a set of core symptoms at specified frequency and severity levels.

A Validated Scale for Assessing the Severity of Acute Infectious Mononucleosis

To develop a scale for the severity of mononucleosis. One to 5 percent of college students develop infectious mononucleosis annually, and about 10% meet criteria for chronic fatigue syndrome (CFS) 6months following infectious mononucleosis. We developed a severity of mononucleosis scale based on a review of the literature. College students were enrolled, generally when they were healthy. When the students developed infectious mononucleosis, an assessment was made as to the severity of their infectious mononucleosis independently by 2 physicians using the severity of mononucleosis scale. This scale was correlated with corticosteroid use and hospitalization. Six months following infectious mononucleosis, an assessment is made for recovery from infectious mononucleosis or meeting 1 or more case definitions of CFS. In total, 126 severity of mononucleosis scales were analyzed. The concordance between the 2 physician reviewers was 95%. All 3 hospitalized subjects had severity of mononucleosis scores ≥2. Subjects with severity of mononucleosis scores of ≥1 were 1.83 times as likely to be given corticosteroids. Students with severity of mononucleosis scores of 0 or 1 were less likely to meet more than 1 case definition of CFS 6months following infectious mononucleosis. The severity of mononucleosis scale has interobserver, concurrent and predictive validity for hospitalization, corticosteroid use, and meeting criteria for CFS 6months following infectious mononucleosis.

The Development of the DePaul Symptom Questionnaire: Original, Expanded, Brief, and Pediatric Versions.

One of the key requirements of a reliable case definition is the use of standardized procedures for assessing symptoms. This article chronicles the development of the DePaul Symptom Questionnaire (DSQ) to assess symptoms of the major chronic fatigue syndrome (CFS) and myalgic encephalomyelitis (ME) case definitions. The original questionnaire has been modified and expanded over time to more fully capture symptoms from various adult case definitions, and a brief as well as pediatric version have also been developed. The DSQ has demonstrated very good psychometric properties in terms of test-retest reliability and sensitivity/specificity, as well as construct, predictive, and discriminant validity. The DSQ allows for a clear characterization of a patient's illness and allows scientists and clinicians to improve diagnostic reliability and validity when employing case definitions of ME and CFS.

The development of an instrument to assess post-exertional malaise in patients with myalgic encephalomyelitis and chronic fatigue syndrome.

Post-exertional malaise, or a variation of this term, is a key symptom of myalgic encephalomyelitis and chronic fatigue syndrome, as this symptom is mentioned in almost all myalgic encephalomyelitis and chronic fatigue syndrome case definitions. Until now there has not been a comprehensive questionnaire to assess post-exertional malaise. To rectify this situation, in this article we describe the development of a new questionnaire, called the DePaul Post-Exertional Malaise Questionnaire, which was based on input from hundreds of patients. Preliminary validation was provided by the findings of significant and predictable relationships between different domains of this post-exertional malaise questionnaire and physical functioning.

Patient perceptions of post exertional malaise

ABSTRACT Background: One of the cardinal symptoms of Myalgic Encephalomyelitis (ME) and Chronic Fatigue Syndrome (CFS) is post-exertional malaise (PEM). Almost all ME and CFS case definitions mention this symptom, and there have been several efforts to measure it with self-report items. However, controversy has surrounded how patients feel about the measurement of this symptom. Purpose: The current study presents data on a large patient sample concerning their feelings regarding different aspects of PEM. Results: Findings indicated that the majority of patients did support certain wordings of both the precipitants and consequences of PEM. In addition, considerable approval was also found for a number of ways to phrase items assessing PEM. Overall, findings indicated that screening items from the DePaul Symptom Questionnaire (DSQ) were able to identify 97% of patients, which was higher than any other item. However, the PEM question that was most acceptable from patients was from Ramsay, but other highly endorsed questions were from the ME-ICC and DSQ. Conclusions: As there has not yet been an instrument constructed specifically to measure the complexity of this symptom, these results could be used to develop a prototype for such a questionnaire.

Approaching recovery from myalgic encephalomyelitis and chronic fatigue syndrome: Challenges to consider in research and practice.

There are unique methodological challenges to studying and assessing recovery in myalgic encephalomyelitis and chronic fatigue syndrome. This study explored these challenges through interviewing 13 physicians who treat myalgic encephalomyelitis and chronic fatigue syndrome. Our deductive thematic analysis produced four themes to consider when approaching recovery: lifespan differences in the illness experience; the heterogeneity of myalgic encephalomyelitis and chronic fatigue syndrome-case definitions, etiological stance, and misdiagnosis; patient follow-up and selection bias; and assessment logistics. We discuss how researchers and clinicians can use these considerations when working with patients, drafting recovery criteria, and interpreting treatment outcomes.

Altered neuroendocrine control and association to clinical symptoms in adolescent chronic fatigue syndrome: a cross-sectional study.

BackgroundChronic fatigue syndrome (CFS) is a common and disabling disorder, and a major threat against adolescent health. The pathophysiology is unknown, but alteration of neuroendocrine control systems might be a central element, resulting in attenuation of the hypothalamus–pituitary–adrenalin (HPA) axis and enhancement of the sympathetic/adrenal medulla (SAM) system. This study explored differences in neuroendocrine control mechanisms between adolescent CFS patients and healthy controls, and whether characteristics of the control mechanisms are associated with important clinical variables within the CFS group.MethodsCFS patients 12–18 years of age were recruited nation-wide to a single referral center as part of the NorCAPITAL project. A broad case definition of CFS was applied. A comparable group of healthy controls were recruited from local schools. A total of nine hormones were assayed and subjected to network analyses using the ARACNE algorithm. Symptoms were charted by a questionnaire, and daily physical activity was recorded by an accelerometer.ResultsA total of 120 CFS patients and 68 healthy controls were included. CFS patients had significantly higher levels of plasma norepinephrine, plasma epinephrine and plasma FT4, and significantly lower levels of urine cortisol/creatinine ratio. Subgrouping according to other case definitions as well as adjusting for confounding factors did not alter the results. Multivariate linear regression models as well as network analyses revealed different interrelations between hormones of the HPA axis, the SAM system, and the thyroid system in CFS patients and healthy controls. Also, single hormone degree centrality was associated with clinical markers within the CFS group.ConclusionThis study reveals different interrelation between hormones of the HPA axis, the SAM system, and the thyroid system in CFS patients and healthy controls, and an association between hormone control characteristics and important clinical variables in the CFS group. These results add to the growing insight of CFS disease mechanisms.Trial registration Clinical Trials NCT01040429

Unintended Consequences of not Specifying Exclusionary Illnesses for Systemic Exertion Intolerance Disease.

The Institute of Medicine recently proposed a new case definition for chronic fatigue syndrome (CFS), as well as a new name, Systemic Exertion Intolerance Disease (SEID). Contrary to the Fukuda et al.’s CFS case definition, there are few exclusionary illnesses specified for this new SEID case definition. The current study explored this decision regarding exclusionary illnesses using the SEID criteria with four distinct data sets involving patients who had been identified as having CFS, as well as healthy controls, community controls, and other illness groups. The findings indicate that many individuals from major depressive disorder illness groups as well as other medical illnesses were categorized as having SEID. The past CFS Fukuda et al. prevalence rate in a community based sample of 0.42 increased by 2.8 times with the new SEID criteria. The consequences for this broadening of the case definition are discussed.

Treatment of Myalgic Encephalomyelitis/Chronic Fatigue Syndrome: A Systematic Review for a National Institutes of Health Pathways to Prevention Workshop.

Myalgic encephalomyelitis (ME)/chronic fatigue syndrome (CFS) is a debilitating multisystem condition affecting more than 1 million adults in the United States. To determine benefits and harms of treatments for adults with ME/CFS and identify future research needs. MEDLINE, PsycINFO, and Cochrane databases (January 1988 to September 2014); clinical trial registries; reference lists; and manufacturer information. English-language randomized trials of the effectiveness and adverse effects of ME/CFS treatments. Data on participants, study design, analysis, follow-up, and results were extracted and confirmed. Study quality was dual-rated by using prespecified criteria; discrepancies were resolved through consensus. Among 35 treatment trials enrolling participants primarily meeting the 1994 Centers for Disease Control and Prevention and Oxford case definitions of CFS, the immune modulator rintatolimod improved some measures of exercise performance compared with placebo in 2 trials (low strength of evidence). Trials of galantamine, hydrocortisone, IgG, valganciclovir, isoprinosine, fluoxetine, and various complementary medicines were inconclusive (insufficient evidence). Counseling therapies and graded exercise therapy compared with no treatment, relaxation, or support improved fatigue, function, global improvement, and work impairment in some trials; counseling therapies also improved quality of life (low to moderate strength of evidence). Harms were rarely reported across studies (insufficient evidence). Trials were heterogeneous and were limited by size, number, duration, applicability, and methodological quality. Trials of rintatolimod, counseling therapies, and graded exercise therapy suggest benefit for some patients meeting case definitions for CFS, whereas evidence for other treatments and harms is insufficient. More definitive studies comparing participants meeting different case definitions, including ME, and providing subgroup analysis are needed to fill research gaps.

Chronic fatigue syndrome versus systemic exertion intolerance disease

Background: The Institute of Medicine has recommended a change in the name and criteria for chronic fatigue syndrome (CFS), renaming the illness systemic exertion intolerance disease (SEID). The new SEID case definition requires substantial reductions or impairments in the ability to engage in pre-illness activities, unrefreshing sleep, post-exertional malaise, and either cognitive impairment or orthostatic intolerance. Purpose: In the current study, samples were generated through several different methods and were used to compare this new case definition to previous case definitions for CFS, the International Consensus Criteria for myalgic encephalomyelitis (ME-ICC), the Canadian myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) definition, as well as a case definition developed through empirical methods. Methods: We used a cross-sectional design with samples from tertiary care settings, a BioBank sample, and other forums. Seven hundred and ninety-six patients from the USA, Great Britain, and Norway completed the DePaul Symptom Questionnaire. Results: Findings indicated that the SEID criteria identified 88% of participants in the samples analyzed, which is comparable to the 92% that met the Fukuda criteria. The SEID case definition was compared to a four-item empiric criteria, and findings indicated that the four-item empiric criteria identified a smaller, more functionally limited and symptomatic group of patients. Conclusion: The recently developed SEID criteria appears to identify a group comparable in size to the Fukuda et al. criteria, but a larger group of patients than the Canadian ME/CFS and ME criteria, and selects more patients who have less impairment and fewer symptoms than a four-item empiric criteria.

Chronic fatigue syndrome and fibromyalgia in diagnosed sleep disorders: a further test of the 'unitary' hypothesis.

BackgroundSince chronic fatigue syndrome (CFS) and fibromyalgia (FM) often co-exist, some believe they reflect the same process, somatization. Against that hypothesis are data suggesting FM but not CFS was common in patients with sleep-disordered breathing (SDB). The presence of discrete case definitions for CFS and FM allowed us to explore rates of CFS alone, CFS with FM, and FM alone in SDB patients compared to those with sleep complaints that fulfilled criteria for insomnia.MethodsParticipants were 175 sequential patients with sleep-related symptoms (122 had SDB and 21 had insomnia) and 39 healthy controls. Diagnoses were made by questionnaires, tender point count, and rule out labs; sleepiness was assessed with Epworth Sleepiness Scale and mood with Beck Depression Inventory.ResultsRates of CFS, FM or CFS + FM were high: 13% in SDB and 48% in insomnia. CFS occurred frequently in SDB and insomnia, but FM occurred frequently only in insomnia. SDB patients with CFS and/or FM had higher daytime sleepiness than those without these disorders.ConclusionCFS patients should complete Epworth scales, and sleep evaluation should be considered for those with scores ≥ 16 before receiving the diagnosis of CFS; the coexistence of depressed mood in these patients suggests some may be helped by treatment of their depression. That FM was underrepresented in SDB suggests FM and CFS may have different underlying pathophysiological causes.

An Overview of Operationalizing Criteria for ME, ME/CFS, and CFS Case Definitions

Since the 1950s, many case definitions have been developed for chronic fatigue syndrome (CFS) (Holmes etal., 1988; Fukuda etal., 1994), Myalgic Encephalomyelitis (ME) (Carruthers etal., 2011; Ramsa...

Plasma cytokine expression in adolescent chronic fatigue syndrome.

Chronic fatigue syndrome (CFS) is a prevalent and disabling condition among adolescents. The pathophysiology is poorly understood, but low-grade systemic inflammation has been suggested as an important component. This study compared circulating levels of individual cytokines and parameters of cytokine networks in a large set of adolescent CFS patients and healthy controls, and explored associations between cytokines and symptoms in the CFS group. CFS patients (12-18years old) were recruited nation-wide to a single referral center as part of the NorCAPITAL project (ClinicalTrials ID: NCT01040429). A broad case definition of CFS was applied, requiring three months of unexplained, disabling chronic/relapsing fatigue of new onset, whereas no accompanying symptoms were necessary. Thus, the case definition was broader than the Fukuda-criteria of CFS. Healthy controls having comparable distribution of gender and age were recruited from local schools. Twenty-seven plasma cytokines, including interleukins, chemokines and growth factors were assayed using multiplex technology. The results were subjected to network analyses using the ARACNE algorithm. Symptoms were charted by a questionnaire, and patients were subgrouped according to the Fukuda-criteria. A total of 120 CFS patients and 68 healthy controls were included. CFS patients had higher scores for fatigue (p<0.001) and inflammatory symptoms (p<0.001) than healthy controls. All cytokine levels and cytokine network parameters were similar, and none of the differences were statistically different across the two groups, also when adjusting for adherence to the Fukuda criteria of CFS. Within the CFS group, there were no associations between aggregate cytokine network parameters and symptom scores. Adolescent CFS patients are burdened by symptoms that might suggest low-grade systemic inflammation, but plasma levels of individual cytokines as well as cytokine network measures were not different from healthy controls, and there were no associations between symptoms and cytokine expression in the CFS group. Low-grade systemic inflammation does not appear to be a central part of adolescent CFS pathophysiology.

A definition of recovery in myalgic encephalomyelitis and chronic fatigue syndrome should be based upon objective measures.

Adamowicz and colleagues recently proposed to use "a consistent definition of recovery that captures a broad-based return to health with assessments of both fatigue and function as well as the patients' perceptions of his/her recovery status" for patients with chronic fatigue syndrome (CFS). A qualitative analysis of case definitions for Myalgic encephalomyelitis (ME) and CFS and methods to assess the symptoms and clinical status of ME and CFS patients objectively. The criteria of CFS define a heterogeneous disorder. ME, often used interchangeably with CFS, is principally defined by muscle weakness, cognitive impairment etc., but above all post-exertional "malaise": a long-lasting increase in symptoms, e.g. muscle pain and cognitive deficits, after a minor exertion. The principle symptom of CFS however is "chronic fatigue". Since post-exertional "malaise" is not obligatory for CFS, only part of the CFS patients meet the diagnostic criteria for ME, while not all ME patients qualify as CFS patients. There are several accepted methods to assess characteristic symptoms and the clinical status of ME and CFS patients using objective measures, e.g. (repeated) cardiopulmonary exercise tests. To resolve the debate about the clinical status, proposed effectiveness of therapies and recovery in ME and CFS, it is crucial to accurately diagnose patients using well-defined criteria for ME and CFS and an objective assessment of various typical symptoms, since subjective measures such as "fatigue" will perpetuate the debate.

Identifying Defining Aspects of Chronic Fatigue Syndrome via Unsupervised Machine Learning and Feature Selection

In this work we propose an unsupervised machine learning method of predicting chronic fatigue syndrome (CFS) based on the k-means algorithm using self-reported questionnaire responses. We first suggest a method of determining the presence of a symptom based on its frequency and severity using an unsupervised dynamic thresholding approach. This threshold is used to diagnose subjects with 54 symptoms related to CFS. Based on these diagnoses, k-means is used to predict the presence of CFS. We find that k-means does not have significantly worse predictive diagnostic accuracy than commonly used CFS case definitions. After applying supervised feature selection, k-means achieves significantly better diagnostic accuracy than any of the case definitions examined. We use these results to suggest the basis for an empirically founded CFS case definition.

Examining case definition criteria for chronic fatigue syndrome and myalgic encephalomyelitis

Background: Considerable controversy has transpired regarding the core features of myalgic encephalomyelitis (ME) and chronic fatigue syndrome (CFS). Current case definitions differ in the number and types of symptoms required. This ambiguity impedes the search for biological markers and effective treatments. Purpose: This study sought to empirically operationalize symptom criteria and identify which symptoms best characterize the illness. Methods: Patients (n = 236) and controls (n = 86) completed the DePaul Symptom Questionnaire, rating the frequency and severity of 54 symptoms. Responses were compared to determine the threshold of frequency/severity ratings that best distinguished patients from controls. A Classification and Regression Tree (CART) algorithm was used to identify the combination of symptoms that most accurately classified patients and controls. Results: A third of controls met the symptom criteria of a common CFS case definition when just symptom presence was required; however, when frequency/severity requirements were raised, only 5% met the criteria. Employing these higher frequency/severity requirements, the CART algorithm identified three symptoms that accurately classified 95.4% of participants as patient or control: fatigue/extreme tiredness, inability to focus on multiple things simultaneously, and experiencing a dead/heavy feeling after starting to exercise. Conclusions: Minimum frequency/severity thresholds should be specified in symptom criteria to reduce the likelihood of misclassification. Future research should continue to seek empirical support of the core symptoms of ME and CFS to further progress the search for biological markers and treatments.

Contrasting chronic fatigue syndrome versus myalgic encephalomyelitis/chronic fatigue syndrome

Background: Much debate is transpiring regarding whether chronic fatigue syndrome (CFS) and myalgic encephalomyelitis (ME) are different illnesses. Several prior studies that compared the Fukuda et al. CFS criteria to the Canadian ME/CFS criteria found that the Canadian criteria identified patients with more functional impairments and greater physical, mental, and cognitive problems than those who met Fukuda et al. criteria. These samples were located in the Chicago metropolitan area, so the results could not be generalized to other locations. In addition, past studies used a symptom questionnaire that was not specifically developed to tap the Canadian criteria. Purpose: The present comparative study of CFS and ME/CFS criteria was intended to correct the limitations of prior studies. Methods: This article used data from three distinct samples to compare patients who met criteria for the ME/CFS Canadian clinical case definition to those who met the Fukuda et al. CFS case definition. Results: Findings indicated that fewer individuals met the Canadian criteria than the Fukuda et al. criteria. Those who met the Canadian criteria evidenced more severe symptoms and physical functioning impairment. Conclusions: Future research should continue to compare existing case definitions and determine which criteria best select for this illness.