Definition: a temperature >38.3 C (101 F) for at least three weeks’ duration AND failure to identify a source after one week of appropriate medical workup.
Approach to the Adult Patient with Fever of Unknown Origin. ALAN R. ROTH, D.O., and GINA M. BASELLO, D.O., Jamaica Hospital Medical Center, Mount Sinai School of Medicine Family Practice Residency Program, Jamaica, New York. Am Fam Physician. 2003 Dec 1;68(11):2223-2229.