Further validation was undertaken comparing the groups using age, sex, illness duration and whether they met the CDC or London criteria for CFS (The National Task Force on Chronic Fatigue Syndrome, 1994; Reeves et al. 2003) (Table 4).
Of the patients, 62% met the CDC criteria for CFS, but differences across subgroups were not statistically significant (p = 0.12). In contrast, 53% of all patients met the London criteria for ME, and differences across subgroups could not be explained by chance alone (p < 0.001). This relationship, however, appeared to be driven by the low proportion of patients in the mood and polysymptomatic groups.This was to be expected, given that the high levels of co-morbid anxiety and depression that characterized these subgroups would, by definition, result in exclusion from the London criteria (White et al. 2007).
There was a significant difference between subgroups for sex (p = 0.014), with the FSS group, and to
a lesser extent avoidant–inactive, having a relatively greater proportion of women. Age and illness duration did not differ across subgroups (data not shown).