The Development of a Revised Canadian Myalgic Encephalomyelitis Chronic Fatigue Syndrome Case Definition

Problem statement: Several investigators have indicated that case definitions for Chronic Fatigue Syndrome (CFS) are characterized by vaguely worded criteria that lack operational definitions and guidelines. The most widely used CFS case definition is the Fukuda et al. criteria, which uses polythetic criteria (i.e., patients are only required to have four out of a possible eight symptoms). Yet two of these eight symptoms (post-exertional malaise and memory/concentration problems) are an essential feature of this illness and the Fukuda et al. criteria do not require that these symptoms be present among all patients. Significant methodological problems could occur if investigators in different settings recruit samples with different percentages of these core symptoms. In contrast, the Canadian clinical case definition does require specific ME/CFS symptoms such as post-exertional malaise and memory/concentration problems. The provision of operationally explicit, objective criteria on specific key symptoms might reduce criterion variance as a source of unreliability. In addition, the use of structured interview schedules will ensure that symptoms are assessed in a consistent way across settings. Conclusion/Recommendations: In this article, we specified explicit rules for determining whether critical symptoms meet ME/CFS criteria using a revised Canadian case definition and a questionnaire has been developed to assess core symptoms. It is hoped that these developments will lead to increased reliability of this revised Canadian case definition as well as more frequent use of these criteria by investigators.