Screening for prolonged fatigue syndromes: validation of the SOFA scale
Background: The identification of syndromes characterised by persistent and disabling mental and/or physical fatigue is of renewed interest in psychiatric epidemiology. This report details the development of two specific instruments: the SOFA/CFS for identification of patients with chronic fatigue syndrome (CFS) in specialist clinics and the SOFA/GP for identification of prolonged fatigue syndromes (PFS) in community and primary care settings. Methods: Patients with clinical diagnoses of CFS (n = 770) and consecutive attenders at primary care (n = 1593) completed various self-report questionnaires to assess severity of current fatigue-related symptoms and other common somatic and psychological symptoms. Quality receiver operating characteristic curves were used to derive appropriate cut-off scores for each of the instruments. Comparisons with other self-report measures of anxiety, depression and somatic distress are noted. Various multivariate statistical modelling techniques [latent class analysis (LCA), longitudinal LCA] were utilised to define the key features of PFS and describe its longitudinal characteristics. Results: The SOFA/CFS instrument performs well in specialist samples likely to contain a high proportion of patients with CFS disorders. Cut-off scores of either 1/2 or 2/3 can be used, depending on whether the investigators wish to preferentially emphasise false-negatives or false-positives. Patients from these settings can be thought of as consisting not only of those with a large number of unexplained medical symptoms, but also those with rather specific musculoskeletal and pain syndromes. The SOFA/GP instrument has potential cut-off scores of 1/2 or 2/3, with the latter preferred as it actively excludes all non-PFS cases (sensitivity = 81%, specificity = 100%). Patients with these syndromes in the community represent broader sets of underlying classes, with the emergence of not only musculoskeletal and multisymptomatic disorders, but also persons characterised by significant cognitive subjective impairment. Twelve-month longitudinal analyses of the primary care sample indicated that the underlying class structure was preserved over time. Comparisons with other measures of psychopathology indicated the relative independence of these constructs from conventional notions of anxiety and depression. Conclusions: The SOFA/GP instrument (which is considerably modified from the SOFA/CFS in terms of anchor points for severity and chronicity) is preferred for screening in primary care and community settings. Patients with PFS and CFS present a range of psychopathology that differs in its underlying structure, cross-sectionally and longitudinally, from coventional notions of anxiety and depression.
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