We see no reason to discard the term fever of unknown origin, defined in 1961 and familiar to generations of internists by its acronymFUO.1 It represents one of the most challenging diagnostic problems, addressed in every medical textbook and in leading medical journals.2-4 Criteria for case definition are a matter of debate in every field of medicine, yet they are essential for meaningful comparison between different series, over time, between regions, and between different types of hospitals.4 Most investigators accept the criteria of duration of illness and the degree of temperature elevation. The third criterion (duration of investigation) is more controversial, and a qualitative criterion, consisting of a set of baseline investigations, has been proposed.3-5 The term FUO should be used with respect to a rigid, updated definition and with diagnostic humility.3 Fever of unknown origin is not a disease but a clinical problem of prolonged fever that defies an initial intelligent approach.
Vanderschueren S, Knockaert D. Why Do We Still Use the Term FUO?—Reply. Arch Intern Med. 2003;163(17):2102–2103. doi:10.1001/archinte.163.17.2102-a
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