Inflammation of unknown origin versus fever of unknown origin: Two of a kind

ObjectivesA vast literature exists on fever of unknown origin (FUO), characterized by prolonged and perplexing fevers > 38.3 °C. In contrast, no studies are available to guide the approach to inflammation of unknown origin (IUO), defined as prolonged and perplexing inflammation with temperatures < 38.3 °C. We aimed to determine the diagnostic yield, the case-mix, and the outcome of patients with IUO, relative to patients with FUO.MethodsWe matched 57 patients with IUO to 57 patients with FUO of the same gender (54% male) and a similar age (median: 67 years).ResultsA diagnosis was established in 35 patients with IUO (61%) and in 33 patients with FUO (58%) (p = .70). The case-mix did not differ significantly (p = .43). Non-infectious inflammatory disorders were the dominant diagnostic category in the IUO group (16 patients), while in the FUO group, similar numbers of malignancies [10], infections [9], and non-infectious inflammatory diseases [9] were diagnosed. 18F-fluorodeoxyglucose-positron emission tomography (FDG-PET) scan contributed comparably to the diagnosis in both groups (in 18 of 50, 36%, patients with IUO and in 13 of 40, 33%, patients with FUO) (p = .83). In both groups, 7 patients (12%) died during an average follow-up of 1 year.ConclusionDiagnostic yield, case-mix, contribution of FDG-PET scan and vital outcome were similar in patients with IUO and FUO. These data suggest that the 38.3 °C boundary may be arbitrary and that the diagnostic approaches used in FUO can be applied to IUO.