18F-fluorodeoxyglucose positron emission tomography (18F-FDG PET) imaging often plays an important role in the workup of fever of unknown origin (FUO) and inflammation of unknown origin (IUO), especially in those with few or no diagnostic clues. FUO is defined as an illness duration of ≥3 weeks with fever ≥38.3 °C on ≥3 occasions and no diagnosis despite baseline investigations. IUO, which was first described in 2009, is defined similarly but instead of high-grade fevers these patients have ≥3 occasions of elevated inflammatory markers, for which there is currently no universally accepted cut-off value [1–4].
