Clinical differences exist between patients fulfilling the 1988 and 1994 case definitions of chronic fatigue syndrome

Despite the existence of 2 different case definitions for chronic fatigue syndrome, little data exist to evaluate how each performs. We evaluated the symptom patterns of patients fulfilling either the more demanding 1988 or the less demanding 1994 case definitions of Chronic Fatigue Syndrome over an 11-year period from 1991 to 2001. Factor analysis identified 3 factors: muscular-skeletal, viral, and sleep/memory symptom factors accounting for significant variation in the data. Further discriminant analysis showed that the first 2 factors and a self-reported measure of decrease in activity alone gave 91% accuracy in the placement of patients into their respective case definitions. This analysis did indicate that both case definitions were capturing the same broad group of patients. However, the patients in the 1994 group do not endorse infectious-type symptoms as often or to the same degree of severity as those in the 1988 group. This may mean that infection as a cause of CFS is more likely in patients fulfilling the earlier, more demanding case definition.