A two-day CPET cannot be used to rule out chronic fatigue syndrome. : 216

Definitions
Notable definitions include:
Centers for Disease Control and Prevention (CDC) definition (1994), the most widely used clinical and research description of CFS, is also called the Fukuda definition and is a revision of the Holmes or CDC 1988 scoring system. The 1994 criteria require the presence of four or more symptoms beyond fatigue, while the 1988 criteria require six to eight. The ME/CFS 2003 Canadian Clinical working definition states: "A patient with ME/CFS will meet the criteria for fatigue, post-exertional malaise and/or fatigue, sleep dysfunction, and pain; have two or more neurological/cognitive manifestations and one or more symptoms from two of the categories of autonomic, neuroendocrine, and immune manifestations; and the illness persists for at least 6 months". The Myalgic Encephalomyelitis International Consensus Criteria (ICC) published in 2011 is based on the Canadian working definition  and has an accompanying primer for clinicians The ICC does not have a six months waiting time for diagnosis. The ICC requires post-exertional neuroimmune exhaustion (PENE) which has similarities with post-exertional malaise, plus at least three neurological symptoms, at least one immune or gastrointestinal or genitourinary symptom, and at least one energy metabolism or ion transportation symptom. Unrefreshing sleep or sleep dysfunction, headaches or other pain, and problems with thinking or memory, and sensory or movement symptoms are all required under the neurological symptoms criterion. According to the ICC, patients with post-exertional neuroimmune exhaustion but only partially meet the criteria should be given the diagnosis of atypical myalgic encephalomyelitis.
In February 2010, the All-Party Parliamentary Group on ME (APPG on ME) produced a legacy paper, which welcomed the recent MRC initiative, but felt that  far too much emphasis in the past had been on psychological research, with insufficient attention to biomedical research, and that further biomedical research must be undertaken to help discover a cause and more effective forms of management for this disease.Controversy surrounds psychologically oriented models of the disease and behavioral treatments conducted in the UK. United States
In 1998, $13 million  for CFS research was found to have been redirected or improperly accounted for by the United States CDC, and officials at the agency misled Congress about the irregularities. The agency stated that they needed the funds to respond to other public-health emergencies. The director of a US national patient advocacy group charged the CDC had a bias against studying the disease. The CDC pledged to improve their practices and to restore the $13 million to CFS research over three years.On 29 October 2015, the National Institutes of Health declared its intent to increase research on ME/CFS. The NIH Clinical Center was to study individuals with ME/CFS, and the National Institute of Neurological Disorders and Stroke would lead the Trans-NIH ME/CFS Research Working Group as part of a multi-institute research effort.