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Fever of unknown origin (FUO) has been recognized as a disease state for over
100 years. The first commonly accepted definition of FUO was published in 1961
and included (1) fever greater than 101 F (38.3 C) on multiple occasions (2) at least
3 week duration of fever (3) without identifiable cause despite at least a 1 week inpa-
tient evaluation.1 Overtime, this definition was considered both too rigid and too ill
defined. Given the impracticality of prolonged hospitalizations and advancing technol-
ogy for diagnosis, the duration of work up was shortened to 3 days inpatient evaluation
and/or 3 outpatient clinic visits.2

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这是FUO(不明原因发烧)的综合指南，第一个普遍接受的 FUO 定义是1961 年发布的，包括多次发烧超过三十八度三，发烧持续时间至少 3 周，并且就算住院至少 1 周但仍无明确原因。后面这个定义被认为过于僵化且过于模糊。又改成了三天住院评估和/或三次门。除了有关评估持续时间的所有时间要求，还应该将 FUO 定义为完成最低限度检查后缺乏已知病因的发烧。

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常见原因就感染性，自身免疫和结缔组织疾病，其他炎症过程和恶性肿瘤。结缔组织疾病和自身免疫性疾病是青少年 FUO 比儿童更常见的原因。
总体上有超过两百种病因可导致 FUO

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