CO39 | The FADOI-SIMIT Italian register of fever of unknown origins

Background and Aims of the study: Fever of unknown origin (FUO) is a rare syndrome, but it has relevant morbidity, diagnostic complexity and high healthcare costs. From 20% to 50% of FUO cases remain with no diagnosis. Through a national register, a collaboration between the FADOI and SIMIT societies, this prospective study enrolled FUO cases observed in adult or hospitalized patients, or patients evaluated in 28 Italian centers (15 Infective Disease Units, and 13 Internal Medicine Units).Materials and Methods: The fever (≥38.3°C) had to be found in more than one occasion and last >3 weeks, with no diagnosis after the first level exams. The register was active from 2019, October 1, to 2023, June 30. The study included a 6-month follow-up. We excluded HIV-positive, or neutropenic patients, or patients who were hospitalized for other pathologies. One hundred and eighty-seven patients were enrolled: 51% were women, with an average age of 54 (18-96 range); 40% with no comorbidities. Among the 177 patients who were ultimately evaluated, 28% received no diagnosis, 15% a possible diagnosis, and 57% a conclusive diagnosis. Causes include infections (38%), rheumatological (41%), neoplastic (14%), and other causes (7%). PET-CT was made on 65% of patients, leading to diagnosis in 41% of cases. A diagnosis was made after a median 23 days from enrollment; 65% of patients with a conclusive diagnosis had healed by the time of the follow-up.Conclusions: This register gives an up-to-date insight of FUO causes, their evolution, and the diagnostic paths used in Italy.