Fever of unknown origin (FUO) occurs across all age groups. However, age, together with epidemiologic considerations, are important factors in forming a differential diagnosis.
In 1961, the first formal criteria for FUO were published, based on a prospective trial.[1] The criteria defined FUO as a fever higher than 100.9°F (38.3°C) on several occasions, persisting without a diagnosis for at least three weeks, despite having investigations for at least one week in hospital.[1]
With subsequent advancements in medical care, a revised definition for classic FUO was published in 1991, including a proviso for patients with an uncertain diagnosis despite three days in hospital or three outpatient visits.[2] The revised definition also outlined three additional groups of FUO: nosocomial (healthcare-associated), neutropenic (immune-deficient), and HIV-related.
In 1997, a prospective study advised replacing the time-based criteria with a standardized diagnostic protocol to minimize differences in diagnostic workup and unintended biases that could influence outcomes.[3] This FUO definition is now termed the qualitative FUO criteria, and is detailed in the Diagnostic approach .[3] [4]
