What were the characteristics of the patient cohort? Angela Kennedy, N/a 22 February 2010 Immediate questions that spring to mind relate to the patient cohort and whether they were similar enough to the patient cohort in the Lombardi et al project. The Lombardi et al research cohort apparently met criteria for 'ME/CFS' as identified by Carruthers et al, in addition to Fukuda et al. which has been demonstrated to select patients with post-exertional malaise and fatigue, sleep dysfunction, pain, clinical neurocognitive, and clinical autonomic/ neuroimmunoendocrine symptoms (Jason et al). Furthermore, the WPI give this information about their patient cohort in their supporting online material: "Their diagnosis of CFS is based upon prolonged disabling fatigue and the presence of cognitive deficits and reproducible immunological abnormalities. These included but were not limited to peturbations of the 2-5A synthetase/RNase L antiviral pathway, low natural killer cell cytotoxicity (as measured by standard diagnostic assyas) and elevated cytokines particularly interleukin-6 and interleukin-8. In addition to these immunological abnormalities, the patients characteristically demonstrated impaired exercise performance with extremely low VO2 max measured on stress testing..." (www.sciencemag.org/cgi/content/full/117905/DC1) The Erlwein et al project selected a rather different patient cohort: ""Patients in our CFS cohort had undergone medical screening to exclude detectable organic illness". In any British subsequent research to Lombardi et al, unless the patient cohort selected was similar enough to that in the Lombardi research, it is premature to believe XMRV is not present in the 'CFS' population in Britain. Ongoing neglect of the importance of establishing a possible ‘CFS’ patient population in Britain, clinically and in research settings, using the Canadian Guidelines, is preventing the development of knowledge that might help extremely ill and disabled people, diagnosed with 'CFS', known to be here in Britain. I am therefore interested if the authors are aware of any serious attempts to identify such a population in Britain, or taking part in such attempts themselves. The problems I have briefly outlined here do not fully express the range and depths of problems with regard to: the identity of an accurate ‘CFS’ population; the instabilities of ‘CFS’ criteria per se; the faulty concepts of ‘medically unexplained’ or ‘functional‘ and relation to ‘psychogenic‘ explanations for somatic illness; the vagaries of criteria that claim to facilitate a ‘diagnosis of exclusion’; and the psychogenic dismissal of serious organic dysfunction of patients given a ’CFS’ diagnosis, problems that have happened for many years. REFERENCES Carruthers, B. et al (2003) “Myalgic Encephalomyelitis/Chronic Fatigue Syndrome: Clinical Working Case Definition, Diagnostic and Treatment Protocols” Journal of Chronic Fatigue Syndrome, Vol. 11(1), pp 7 - 115. Jason LA, Torres-Harding SR, Jurgens A, Helgerson J. “Comparing the Fukuda et al. Criteria and the Canadian Case Definition for chronic Fatigue Syndrome”. Journal of Chronic Fatigue Syndrome 12(1):37-52, 2004 Competing interests Social scientist critically evaluating 'psychogenic' explanations for illnesses such as 'CFS' or 'ME'. Parent of young sufferer of illness given 'CFS' diagnosis.