Background: Ambiguities in case definitions have created difficulties in replicating findings and estimating the prevalence rates for chronic fatigue syndrome (CFS) and Myalgic Encephalomyelitis (ME).
Purpose: The current study examined differences in occurrence rates for CFS and ME cardinal symptoms (i.e. post-exertional malaise, unrefreshing sleep, and neurocognitive deficits).
Results: Findings indicated that there is a wide range of occurrence rates on critical symptoms of the case definition, suggesting that either the types of patients recruited differ in various settings or the questions assessing core symptoms vary in their wording or criteria among different researchers.
Conclusions: The polythetic nature of the case definition may contribute to the wide ranges of symptom occurrence that was found. In order to increase assessed reliability of the symptoms and case definitions, there is a need to better standardize data collection methods and operationalization of symptoms. This solution would reduce the heterogeneity often seen in populations of CFS patients.
Keywords: chronic fatigue syndrome; myalgic encephalomyelitis; operationalization; symptom occurrence.