ABSTRACT
An epidemiological case-definition was developed to distinguish myalgic encephalomyelitis/chronic fatigue syndrome from other chronic fatiguing conditions by evaluating the discriminatory potential of different criteria from previous definitions. A two-part model was derived using consensus and discriminant analytic approaches. The optimal discriminators for the first part were severe debilitating fatigue affecting physical and mental functioning, a reduction in activity to less than 50% of the patient's premorbid activity level, and muscle discomfort (sensitivity 92%, specificity 66%). The variables for the second part included a reduction in activity to less than 50% of the patient's premorbid activity, myalgia, generalized muscle weakness, migratory arthralgia, and swollen lymph nodes (sensitivity 77%, specificity 88%).