There have been relatively few studies assessing the sensitivity and specificity of fatigue scales which are frequently used to identify individuals with chronic fatigue syndrome (CFS) and differentiate them from healthy controls. The present investigation consists of two distinct studies, both of which employ samples of individuals with CFS and controls, and to assess the effectiveness of several well known fatigue instruments in discriminating between these two groups by utilizing Receiver Operating Characteristic (ROC) curve analyses. The Fukuda et al. (1994)
CFS case definition is the currently accepted case definition internationally, although there is no available “gold standard” to assess fatigue severity. This case definition requires an individual to experience six or more months of persisting or recurring chronic fatigue and the co-occurrence of four of eight additional core symptoms. However, these Fukuda et al. requirements have been criticized as lacking operational definitions and guidelines for accurate identification of CFS cases (Jason, King, et al., 1999
; Reeves et al., 2003
). For example, these criteria do not specify how to assess fatigue severity or the presence of persisting or recurring fatigue for a period of 6 or more months. Partially in response to these problems with operationalizing the Fukuda et al. definition, the Centers for Disease Control and Prevention (CDC) developed an empiric case definition for CFS that involves assessment of symptoms, disability, and fatigue with standardized instruments and specific cutoff points (Reeves et al., 2005
The CDC’s empiric CFS case definition (Reeves et al., 2005
) assesses fatigue using the Multidimensional Fatigue Inventory (MFI) (Smets, Garssen, Bonke, & DeHaes, 1995
), a 20-item instrument consisting of several subscales including general fatigue and reduced activity. Reeves et al. define severe fatigue as a score of greater than or equal to 13 on the MFI general fatigue subscale or greater than or equal to 10 on the MFI reduced activity subscale. However, in one study of three groups with CFS, the mean MFI general fatigue scores ranged from 18.3 to 18.8 (Tiersky, Matheis, DeLuca, Lange, & Natelson, 2003
), clearly higher than the Reeves et al. recommended cutoff of 13. In addition, reduced activity scale items refer to issues that a person with depression might easily endorse. For example, a person would meet the fatigue criterion if they stated that the following two items were entirely true: “I get little done,” and “I think I do very little in a day.” It is likely that most individuals with major depressive disorder (MDD) would meet this reduced activity criterion. Jason, Najar, Porter, and Reh (2009)
found that 38% of those with a diagnosis of MDD were misclassified as having CFS using the CDC empiric CFS case definition (Reeves et al.). With the Reeves et al. empiric case definition criteria, the estimated rates of CFS in the US have increased to 2.54% (Reeves et al., 2007
), rates that are about ten times higher than prior CDC prevalence estimates (Reyes et al., 2003
) and estimates of other investigators (Jason, Richman, et al., 1999
). It is plausible that this inflated CFS prevalence estimate in the U.S. is due to an inappropriate broadening of the case definition, and this might very well be related to specificity problems in the measures and cutoffs selected for this case definition.
There have been several reviews of fatigue instruments (Friedberg & Jason, 1998
), some examining scales measuring fatigue intensity alone and others which integrate dimensions of fatigue intensity and functional outcomes associated with fatigue. As an example of a measure of fatigue intensity alone, Chalder et al.’s (1993)
Fatigue Scale is a 14-item verbal rating measure that has strong internal consistency. Using an ROC curve analysis, Jason et al. (1997)
found this scale was able to discriminate a CFS sample from a healthy control sample; however, it was not possible to differentiate the CFS sample from a lupus or multiple sclerosis (MS) sample. The Fatigue Scale was also not able to distinguish between CFS and primary depression (Friedberg & Jason, 2002
), which is a critical diagnostic issue in CFS. In addition, Goudsmit, Stouten, and Howes (2008)
found there was a marked overlap in fatigue scores within a CFS sample between those patients who rated themselves as moderately ill and those who rated themselves as severely ill.
An example of a well known fatigue/function measure is the Fatigue Severity Scale (FSS) (Krupp, LaRocca, Muir-Nash, & Steinberg, 1989
). The FSS is composed of nine items, and in the initial validation study, internal consistency was high for specific illness groups (MS and lupus) and healthy controls. Studies of individuals with CFS revealed that FSS scores were significantly higher for individuals with CFS, as compared to individuals with MS or primary depression (Pepper, Krupp, Friedberg, Doscher, & Coyle, 1993
). Taylor, Jason, and Torres (2000)
compared the FSS with the Fatigue Scale (Chalder et al., 1993
) and found for a CFS-like group, the FSS was more closely associated than the Fatigue Scale with severity ratings for the eight core CFS symptoms (Fukuda et al., 1994
) and a number of functional outcomes. A ceiling effect in the FSS may limit its utility to assess severe fatigue-related disability, and Stouten (2005)
has warned that many fatigue scales do not accurately represent the severe fatigue that is uniquely characteristic of CFS.
Other fatigue scales have also been developed for assessing fatigue severity and related functioning (Fukuda et al., 2008
). For example, the 54-item Profile of Fatigue-Related Symptoms (PFRS) was developed to measure symptomatology specifically related to CFS (Arroll & Senior, 2009
; Ray, Weir, Phillips, & Cullen, 1992
). Each item on the PFRS lists a symptom typical of CFS and respondents are asked to indicate how intensely they have experienced that symptom over the past week. Ray et al. found the following four factors emerged from the PFRS: Emotional Distress, Fatigue, Cognitive Difficulty, and Somatic Symptoms. However, fatigue is a multifaceted construct, and a closer examination of the PFRS Fatigue factor items suggests that many different fatigue states are subsumed in this subscale. For example, items such as “The slightest exercise making you physically tired” and “muscles feeling weak after slight exercise” denotes post-exertional fatigue which relates to the unusual fatigue or malaise experienced by individuals with CFS following exertion. Yet other items within the subscale are more closely related to the experience of energy depletion such as “Feeling physically drained” and “Not having the physical energy to do anything,” indicating inadequate energy reserves which are unrelated to exercise or effort. Consequently, this suggests that fatigue instruments might need to be developed that delineate finer shades of meaning in the context of the physical fatigue experienced by individuals with CFS.
Recently, Jason, Jessen, et al. (2009)
developed the ME/CFS Fatigue Types Questionnaire (MFTQ), a 22-item scale designed to measure the duration, severity and frequency of different fatigue-related sensations and symptoms. Fatigue items encompassed the following dimensions: lack of energy resources needed for daily functioning, over-stimulation of the mind or body without the available energy to act out the mental or physiological excited state, exhaustion or interruption related to everyday cognitive processes, tiredness that is associated with physical symptoms commonly seen in cases of influenza, and abnormal exhaustion following physical activity. Several fatigue factors emerged for individuals with CFS (Post-Exertional, Wired, Brain Fog, Energy, and Flu-Like fatigue), but only one factor emerged for a group of healthy controls. The five-factor structure confirmed in the CFS sample suggests that the symptom of fatigue in this illness is a multi-dimensional entity that is distinct from the generalized form of fatigue experienced by healthy individuals. The MFTQ appears to be a reliable and valid measure of fatigue types in individuals with CFS, but these findings need to be replicated in other CFS samples.
The MFTQ has been compared to other fatigue scales, and some intriguing findings have emerged. For example, Jason, Jessen, et al. (2009)
found a significant correlation between the MFTQ’s Post-Exertional subscale and Ray et al.’s (1992)
PFRS Emotional Distress factor for the control group, but the correlation between these two scales was not significant for the CFS group. These correlations suggest that for the general population, symptoms of post-exertional malaise are significantly related to emotional distress, whereas when people with CFS report symptoms of post-exertional malaise, the symptoms are independent of emotional distress. It is possible that because healthy individuals experience this relationship between emotional distress and post-exertional malaise, they might also believe these two domains are connected for themselves and, by inference, for patients with CFS, when in fact Jason, Jessen, et al.’s findings do not support this.
If fatigue scales are to be recommended for use in the diagnosis of CFS, as has been proposed by Reeves et al. (2005)
, then it is critically important to assess the effectiveness of these scales in differentiating between those with and without CFS. The current investigation sought to assess this critical issue by examining two distinct samples. Study 1 compares a CFS sample with a control group consisting of healthy college students and adults on the following scales: the MFTQ, FSS, the Fatigue Scale, and the PFRS. Study 2 compares a different CFS sample with a control group consisting of adults with a documented diagnosis of an MDD on the following scales: the MFI, Medical Outcomes Study Short Form-36 Health Survey (SF-36; a widely used measure of disability in CFS research), and the CDC Symptom Inventory, the scale recommended by Reeves et al. (2005)
to assess the severity of the core symptoms of CFS. It was hypothesized that the MFTQ (Jason, Jessen, et al., 2009
) would have better sensitivity and specificity than the other commonly used fatigue scales in differentiating between individuals with CFS and controls, as this scale was specifically developed for use with CFS samples and has characteristics that differentiate various aspects of fatigue unique to this illness. The implications of appropriate and inappropriate levels of sensitivity and specificity of these scales are far reaching with regards to obtaining well-characterized research samples. Ultimately, this leads to more ambiguity about the nature of CFS and further stigmatization of individuals disabled by this chronic illness.