Original (1961)3	Temperature ≥ 101°F (38.3°C) on several separate occasions
	Fever lasting longer than three weeks
	Evaluation of at least one week in the hospital
Revised (1991)7	Temperature ≥ 101°F on several separate occasions
	Fever lasting longer than three weeks
	Evaluation of at least three outpatient visits or three days in inpatient care
Qualitative2,6,10–17	Temperature ≥ 101°F documented clinically on several separate occasions
	Appropriate initial diagnostic workup (inpatient or outpatient) does not reveal etiology of fever