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The Chalder Fatigue Scale as an outcome measure
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The outcome measures to be used in the PACE Trial include the Chalder Fatigue Scale (Chalder T, Wessely S
et al. Development of a fatigue scale. J Psychsom Res 1993:37:2:147‐153).
It is important to note that this does not measure the key symptom of ME/CFS (post‐exertional exhaustion
and malaise) but only subjective physical and mental “tiredness” or “fatigue”.
The Chalder Fatigue Scale has been much used by the Wessely School but its validity has been legitimately
questioned.
There are different instruments for scoring symptoms, one being the Likert Scale which has gradations in
measurement, for example, patients can rate themselves on a scale of 1 – 5, and can identify if they feel fine
(score 1), or quite fatigued (score 3), or if they are exhausted (score 5).
The Chalder Fatigue Scale is different; it is a bimodal scale, which essentially means that it has a two‐way
answer only ‐‐ patients must answer simply “yes” or “no” (ie. they are fatigued or they are not fatigued). It
only tells the investigator who is fatigued by the criteria used. It is thus a very crude measurement, because
people with ME/CFS cannot give such clear‐cut answers and are put at a disadvantage: they are not either
“tired” or “not tired” – they vary with different situations.
The Chalder Fatigue Scale makes great claims for the validity of the scale, but it has little, if any, relevance in
ME/CFS, especially where subgroups are concerned, because it lacks sensitivity. It is, however, easy to
analyse.
The Chalder Fatigue Scale appears incapable of providing an accurate assessment of ME/CFS patients as
distinct from fatigued patients. This has been suggested to be because it does not just have a low ceiling for
each individual question, but also for the total score.
In simple terms, if a participant already has a maximum score at the start of an intervention (such as GET),
then even if the participant feels worse and is actually worse at the end of the intervention, their total score
on the Chalder Fatigue scale cannot increase, so there is no evidence that they have been made worse by the
intervention. In other words, people cannot be shown to “get worse” on the Chalder Fatigue Scale even if
they feel ‐‐ and are ‐‐ worse.
Stouten analysed commonly used fatigue scales in relation to “CFS”, including the Chalder Fatigue Scale,
the Checklist Individual Strength and the Krupp fatigue severity. What is clear from this analysis is that the
Chalder Fatigue Scale comes out worst and it did not accurately represent the severe physiological
exhaustion that is characteristic of (ME)CFS, which should lead to serious questions about its validity and
suitability. Abundant extreme scoring and the corresponding inability to discriminate between the various
levels of severe fatigue can produce misleading results in several ways (BMC Health Serv Res 2005:5:37).
Furthermore, Goudsmit et al assessed if there were any problems associated with the Chalder Fatigue Scale
in relation to (ME)CFS patients and found that the low ceiling of the bimodal model means that this scoring
system is not suitable for use in clinical trials (such as the MRC PACE Trial) and that more accurate data
may be obtained using a different instrument (Bulletin of the ICFSME: 2009:16:3).
Since it cannot be used to measure the effect of an intervention, Tom Kindlon from Ireland has correctly and
repeatedly asked why the 11‐item bimodal Chalder Fatigue Scale is being used as a primary outcome
measure in these trials (http://www.biomedcentral.com/1741‐7015/4/9/comments).
The Chalder Fatigue Scale has been described by an Oxford mathematical physicist as “a parody of modern
scientific measurement” (personal communication).