Harrison's Internal Medicine Chapter 19. Fever of Unknown Origin Definition and Classification Fever of unknown origin (FUO) was defined by Petersdorf and Beeson in 1961 as (1) temperatures of °C (101°F) on several occasions; (2) a duration of fever of 3 weeks; and (3) failure to reach a diagnosis despite 1 week of inpatient investigation. While this classification has stood for more than 30 years, Durack and Street have proposed a new system for classification of FUO: (1) classic FUO; (2) nosocomial FUO; (3) neutropenic FUO; and (4) FUO associated with HIV infection. Classic FUO corresponds closely to the earlier. | Chapter 019. Fever of Unknown Origin Part 1 Harrison s Internal Medicine Chapter 19. Fever of Unknown Origin Definition and Classification Fever of unknown origin FUO was defined by Petersdorf and Beeson in 1961 as 1 temperatures of C 101 F on several occasions 2 a duration of fever of 3 weeks and 3 failure to reach a diagnosis despite 1 week of inpatient investigation. While this classification has stood for more than 30 years Durack and Street have proposed a new system for classification of FUO 1 classic FUO 2 nosocomial FUO 3 neutropenic FUO and 4 FUO associated with HIV infection. Classic FUO corresponds closely to the earlier definition of FUO differing only with regard to the prior requirement for 1 week s study in the hospital. The newer definition is broader stipulating three outpatient visits or 3 days in the hospital without elucidation of a cause or 1 week of intelligent and invasive ambulatory investigation. In nosocomial FUO a temperature of C 101 F develops on several occasions in a hospitalized patient who is receiving acute care and in whom infection was not manifest or incubating on admission. Three days of investigation including at least 2 days incubation of cultures is the minimum requirement for this diagnosis. Neutropenic FUO is defined as a temperature of C 101 F on several occasions in a patient whose neutrophil count is 500 pL or is expected to fall to that level in 1-2 days. The diagnosis of neutropenic FUO is invoked if a specific cause is not identified after 3 days of investigation including at least 2 days incubation of cultures. HIV-associated FUO is defined by a temperature of C 101 F on several occasions over a period of 4 weeks for outpatients or 3 days for hospitalized patients with HIV infection. This diagnosis is invoked if appropriate investigation over 3 days including 2 days incubation of cultures reveals no source. Adoption of these categories of FUO in the literature has allowed a more rational .