Measuring improvement and deterioration in myalgic encephalomyelitis/chronic fatigue syndrome: the pitfalls of the Chalder Fatigue Questionnaire

Adamson et al. considered a ≥2-point decrease in Chalder Fatigue Questionnaire score to indicate improvement in fatigue and a ≥2-point increase in Chalder Fatigue Questionnaire score to indicate deterioration in fatigue.¹ While intuitively appealing, data exist that suggest a more complex relationship between changes in Chalder Fatigue Questionnaire scores and clinical change.

Collin and Crawley studied treatment outcomes at 11 specialist myalgic encephalomyelitis/chronic fatigue syndrome clinics in England.² The authors tabulated mean change in Chalder Fatigue Questionnaire score at one year against Clinical Global Impression scores (see additional file 1, table S3). A 2-point decrease in Chalder Fatigue Questionnaire score was reported patients who deemed their health as follows: ‘no change’, ‘a little worse’, ‘much worse’ and ‘very much worse’. The mean changes in Chalder Fatigue Questionnaire score in those categories were similar, with overlapping 95% confidence intervals within the range [−4.77, 2.29]. This suggests that a 2-point decrease in Chalder Fatigue Questionnaire score indicates deterioration or no change in the health of a person with myalgic encephalomyelitis/chronic fatigue syndrome, not improvement.

Adamson et al. report a mean change in Chalder Fatigue Questionnaire score of −6.52, corresponding to the ‘a little better’ category in Collin and Crawley’s data. For comparison, the mean change of those ‘much better’ was an 11-point decrease in Chalder Fatigue Questionnaire score, and ‘very much better’ was a 14.9-point decrease.

Studies using the Chalder Fatigue Questionnaire as a primary outcome measure may miss or underestimate deterioration, because the Chalder Fatigue Questionnaire obscures it. Studies may overestimate improvement if a 2-point decrease on the Chalder Fatigue Questionnaire is used, when a 10-point decrease may be a more appropriate lower bound.

Anchoring one subjective measure to another can hint at cut-offs for clinically important differences, but the use of more objective outcome measures such as actigraphy would be preferable. The unpopularity of such measures among myalgic encephalomyelitis/chronic fatigue syndrome researchers may be linked with the stubborn refusal of more objective outcome measures to budge with current specialist treatment.³