From the Guidelines
Fever of unknown origin (FUO) is defined as a fever higher than 38.3°C (100.9°F) persisting for at least 3 weeks, with no diagnosis despite 3 outpatient visits or in-patient days. This definition is based on the most recent and highest quality study available, which emphasizes the importance of a prolonged period of fever without a clear diagnosis despite appropriate investigation 1. The condition is categorized into four main types: classical, nosocomial, neutropenic, and HIV-related, each with varying etiologies including infectious, inflammatory, malignant, and miscellaneous causes. Key points to consider in the definition and diagnosis of FUO include:
- The duration of fever: at least 3 weeks
- The absence of a clear diagnosis despite investigation
- The categorization into different types based on the patient's condition and setting
- The range of potential causes, from infections to malignancies and autoimmune disorders. The diagnostic approach to FUO involves a thorough history, physical examination, and systematic testing, with the use of tools like [18F]FDG PET/CT showing high diagnostic yield in both FUO and inflammation of unknown origin (IUO) 1.
From the Research
Definition of Fever of Unknown Origin
The definition of fever of unknown origin (FUO) is a disease condition characterized by:
- A temperature exceeding 38.3°C (101°F) on multiple occasions
- Duration of fever for more than three weeks
- No diagnosis made despite a reasonable diagnostic evaluation, which may include:
- The etiology of the fever remains unknown, despite extensive testing and evaluation 2, 3, 5, 4, 6
Key Characteristics
- FUO is often defined as a febrile illness with no obvious source despite appropriate investigation 5
- The differential diagnosis is broad, but typically categorized as:
- Most cases in adults occur due to uncommon presentations of common diseases, and up to 75% of cases may resolve spontaneously without reaching a definitive diagnosis 4