Comparative analysis of inflammation and fever of unknown origin: Diagnostic processes and etiological spectrum

We aimed to evaluate the diagnostic process and etiological spectrum of inflammation of unknown origin (IUO) and fever of unknown origin (FUO). Retrospectively, 8821 patients were evaluated between 2010 and 2024. 168 patients with IUO and 88 with FUO were included in the study. The demographic, clinical, and laboratory characteristics and diagnostic spectra of the IUO and FUO groups were compared. The diagnostic contribution, sensitivity, specificity, and positive and negative predictive values of computed tomography and 18F-FDG PET/CT were calculated. The median age (59 vs 46.7 years; P < .001) and the proportion of males (57.1% vs 40.9%; P = .014) were higher in the IUO. The most common symptom in all patients was arthralgia (78.5%). Weight loss and sore throat were more common in the FUO (P = .012 and P = .004, respectively). c-reactive protein and erythrocyte sedimentation rate levels were similar in both groups. Noninfectious inflammatory diseases (NIID) were the most common cause (31%), followed by neoplasms (18%), infections (11%), and miscellaneous diseases (7%). Infections were significantly more prevalent in the FUO (19.3% vs 6%, P < .001), whereas neoplasms were more commonly observed in the IUO (20.8% vs 12.5%, P = .099). In NIID, vasculitis occurred more frequently in the IUO, whereas adult-onset Still's disease was more prevalent in the FUO (P < .001). 18F-FDG PET/CT contributed 52.1% of cases, with 85% sensitivity and 17% specificity. Infections were common in FUO, whereas patients with IUO were older at presentation, more likely to be male, and had a higher frequency of neoplasms. Among NIIDs, vasculitis was predominant in the IUO, while adult-onset Still's disease was more prominent in the FUO.