As post-exertional malaise is a key symptom of all CFS case definitions, it would be appropriate to measure the extent of activity and how such activity might result in symptoms of fatigue and malaise. Light et al. (2009) found patients with CFS demonstrated increases after exercise that reliably exceeded responses of control subjects in mRNA for genes receptors that can detect muscle produced

metabolites, genes that are essential for sympathetic nervous system processes, and immune function genes. The researchers concluded that CFS patients might have enhanced sensory signal for fatigue that is increased after exercise. Activity, or work performed is generally quantified in terms of energy used, i.e., caloric expenditure. Because this is difficult to measure during activity, total oxygen consumption which increases Seliciclib in a similar fashion, is typically used in its place. Sometimes represented as METs or anti-CTLA-4 antibody metabolic equivalents,

oxygen consumption may be assessed directly using cardiopulmonary exercise testing with measured gas exchange (Milani et al., 2006), or estimated from heart rate or other indicators of effort such as time and/or distance travelled. Assessment of effort is critical when exercise is used as a physiological stressor to elicit symptoms in CFS patients or for assessments of functional capacity as part of clinical trials. Heart rate as a percentage of age-predicted maximum is the most recognized indicator of subject effort for both maximal and submaximal exercise protocols. However, the maximal heart rate response to exercise varies widely in the general population (Balady et al., 2010) and has been shown to be blunted in some subjects with CFS (e.g., VanNess et al., 2003) and also in fibromyalgia (Ribeiro et al., 2011). As an alternative to heart rate, the peak respiratory exchange ratio (RER) is acknowledged as the most valid and reliable gauge of subject effort (Balady et al., 2010). Because it can only be obtained from

ventilatory expired gas analysis, RER may not be available in all exercise studies. Similarly, submaximal exercise protocols do not provide Tenofovir concentration for the measurement of peak RER. In such instances selecting alternative measures that can accurately assess effort both within and across subjects is particularly important. Cognitive impairment is a frequent and troubling symptom in CFS, and optimal objective measures are still being investigated. Biologic measures are increasingly important in studies of CFS. Studies that include any testing need to provide details on the method of specimen collection, transport and processing, as even small deviations may introduce variation. If commercial laboratories are used, the assay method, range of normal values and lower limit of detection should be provided. In house assays need to be described.