Background and Aim : Diagnostic tools for chronic fatigue syndrome, chronic fatigue syndrome questionnaire, created for research and clinical investigations in order to use it as a management tool for monitoring patients with this syndrome. The questionnaire is very comprehensive using a self-reporting system and all internal and external definitions according to the America's Center for Disease Control has been covered. It also assesses current and past activity levels and symptoms of fatigue and malaise after routine daily activities to ensure that these items are evaluated as well. The purpose of this study is to evaluate the reliability and validity of diagnostic questionnaire for Chronic Fatigue Syndrome (CFS).

  Materials and Methods : In the discussion of content validity and reliability of survey studies, chronic fatigue syndrome questionnaire, prepared by the Department of Psychology, DePaul University, and was elected a full copy of the correspondence received from the questionnaire ratings. The questionnaire that was translated content validity was assessed by the 20 members of the faculty. However, re-translated from Persian into English for reference, was prepared with modifications, the validity of its validity was confirmed by the authors. Then, 30 nurses were selected to determine the reliability of the questionnaire chronic fatigue syndrome who was working in a Hospital. Inventory management under center and full justification of the researcher and the participants in the study by researchers at the distribution has been completed. The survey was carried out again after 15 days.

  Results: The instrument showed good reliability and validity. In discussing the content validity of the Persian translation tool by 20 people associated professor of internal review, and its content validity was determined with respect to two of CVI and the results of CVR , and (CVR=0.7 and CVI=0.85) represents CREDIT the content is good . According to the results, the reliability of the questionnaire CFS, Cronbach's alpha to assess the results of the questionnaire showed very good reliability. Two step test reliability coefficients (ICC = Mixed & and Confidence Interval = 95%) also showed a significant relationship among questions as well. However, the internal correlation coefficient obtained for the 8 criteria CFS, showed high internal cohesion.

  Conclusion: This study suggests the CFS questionnaire as a reliable instrument for all levels in research and clinical applications. Using this tool can help with background checks and even in advanced levels to be effective, it has enough power and accuracy in the diagnosis of CFS sufferers.