Fever of unknown origin (FUO) is a fever with no clear cause even after testing for at least 1 to 2 weeks. A fever is often defined as a temperature higher than 101 °F (degrees Fahrenheit) / (38.3 degrees C).
The cause of this fever is unknown. In some people the cause may never be known. Finding a cause may be delayed if there is:
The risk of an illness is often based on the cause. There are no risk factors for FUO since the cause is unknown.
The fever may be steady or come and go. Chills, sweating, and other symptoms may also be present. They can vary based on the cause.
You will be asked about your symptoms and past health. A physical exam will be done. Your doctor may ask questions to look for possible causes. Questions may include:
Tests to look for cause may include:
Medicine may be used to lower fevers that are very high or causing other problems.
Treatment may be changed once a cause is confirmed.
It is hard to prevent FUO since the cause is not known.
Family Doctor—American Academy of Family Physicians
Healthy Children—American Academy of Pediatrics
Caring for Kids—Canadian Paediatric Society
Cunha B, Lortholary O, et al. Review of fever of unknown origin: clinical approach. can be found in Am J Med 2015 Oct;128(10):1138. Accessed at: https://www.ncbi.nlm.nih.gov/pubmed/26093175.
Fever of unknown origin (FUO) in adults. EBSCO DynaMed website. Available at: https://www.dynamed.com/approach-to/fever-of-unknown-origin-fuo-in-adults/. Updated January 13, 2017. Accessed February 6, 2020.
Varghese G, Trowbridge P, et al. Investigating and managing pyrexia of unknown origin in adults. BMJ 2010;341. Accessed at: https://www.ncbi.nlm.nih.gov/pubmed/22312655?dopt=Abstract.
Last reviewed January 2020 by
EBSCO Medical Review Board
David L. Horn, MD, FACP