Primary (or pure) chronic fatigue syndrome (pCFS) is a complex and severe chronic and disabling disease of unknown causes, excluding secondary chronic fatigue syndrome (sCFS) related to some other medical condition. It is characterized by intense fatigue in addition to cognitive, autonomic, neuroendocrine, immunoallergic and musculoskeletal symptoms, which are of recent appearance and that cannot be explained by other clinical reasons, lasting for at least 6 months, is non-remitting significantly with rest and which worsens with physical or mental activity, with very slow recovery and a reduction of >50% of activities of daily living previously performed by the patient.1–4 It is diagnosed according to the 1994 Fukuda criteria,1 the Canadian consensus document published in 20032 or, more recently, the international consensus criteria of 2011; with the name of myalgic encephalomyelitis that offers a review on its physiopathology, symptoms and treatment.3 Prevalence is estimated to be between 0.5% and 2.5% of the general population.1–3,5 In spite of it being recognized as a disease by the WHO since 1989, and classified with the code G93.3 in the ICD-10,5,6 and that evidence accumulated from different fields during the past 2 decades, it is possible that pCFS is still largely unknown by most health professionals.

Due to the great heterogeneity in its clinical expression and the lack of standardized instruments to order its different symptoms (according to greater or lesser frequency) and clinical presentation (distinguishing between pCFS, sCFS and Idiopathic CF), we propose a classification into clusters: I, II, IIIa, IIIb and IIIc, subgrouping the main clinical conditions in a consecutive series of 199 patients (Table 1), seen in a period of time between June 2010 and February 2013 at the Chronic Fatigue Specialized Hospital Unit of the Camp de Tarragona belonging to the Hospital Universitario Joan XXIII. 84% of patients (n=167) were women (5:1). Mean age at symptom onset was 41.5 years (range 9–76). The onset of symptoms was insidious in 72.8% of patients and 71.3% had a progressive evolution of disease. At the moment of interview, 51.7% (n=103) were unemployed (65 lost their job while 38 were disabled). 8% (n=16) were retired. The pCFS criteria are valid for the diagnosis in the clinical daily practice independently of other associated criteria.6–8 Having a classification of patients with CFS will allow us to identify more homogeneous groups of patients, candidates both for more individualized diagnostic and therapeutic guidelines and which allow for better expectations. In order to correctly classify a patient with CFS, we recommend a multidisciplinary approach. In first place, the CFS diagnosis must be confirmed (family physician, internal medicine specialist and/or rheumatologist), in order to then evaluate the existence of possible associated systemic diseases or chronic local processes that have a large differential diagnosis and, finally, to perform a precise diagnosis of the disease with a psychopathological background (psychologist and/or psychiatrist). Having a clear diagnosis in each one of these 6 areas, as well as the analysis of the chronology of symptoms or processes allows the inclusion of the patient into each one of the different classification subgroups.