Fever of unknown origin or, as I put it, "prolonged FUO (≥3 weeks)"1 remains valuable as an operational definition for an unusual yet important and time-consuming clinical problem. Properly understood, it implies both intellectual honesty and an ongoing commitment to the patient. Dr Chang's proposed alternative, "fever of unidentified disease," is inaccurate because fever, a physiologic response, does not always denote disease. It may, however, comfort Dr Chang to know that I have long maintained and taught that frequent use of the modifier cause undetermined (eg, "headache, cause undetermined [suspected migraine variant]") is the mark of the careful and diligent physician.
Bryan CS. Why Do We Still Use the Term FUO?—Reply. Arch Intern Med. 2003;163(17):2103. doi:10.1001/archinte.163.17.2102-a