This study demonstrates some of the construct difficulties in researching this disease.

Myalgic encephalomyelitis (chronic fatigue syndrome) has a number of transdiagnostic symptoms. A common caveat with any medical population is that symptoms that commonly occur with disease, including fatigue or loss of energy, changes in sleep patterns and cognitive difficulties, may be misinterpreted by healthcare providers, researchers or patients as mood-related particularly when specificity and severity are lacking.

As well, many of the instruments used to diagnose mood disorders can become a confounding variable in medical studies. (Jason et al 2012) So might the failure to separate patients with psychosocial overlays from patients without depression or anxiety as well as differentiating between patients with pre-morbid psychiatric illness and patients without.

As well, there are multiple definitions used under these umbrella terms that define very different populations ranging from the very heterogeneous (White et al 1991) to the narrowly defined (Caruthers 2011) some of which may include patients with mood disorders only (Jason et al 2012). This may limit extrapolation of results between groups.

A search of the literature reveals demonstrated cognitive symptoms in ME and CFS including processing-speed deficits and working memory impairment - which are also found in MS for example. (Tiersky et al 1997; DeLuca et al 1997, 2004; Busichio et al 2004; Lange et al 2005).