Fever of Unknown Origin

(FUO; Pyrexia of Unknown Origin)


Fever of unknown origin (FUO) is a higher body temperature with no clear cause, even though there has been at least 1-2 weeks of testing.


The cause of this fever is unknown. In some people the cause may never be known.Factors that may make it difficult to find a cause include:
  • A common illness that does not have the usual symptoms
  • Illness with other symptoms that may appear later
  • Illnesses that may have a delayed positive test
  • Person is unable to communicate about other symptoms such as an infant or someone in a coma
  • Genetic condition that causes periodic fevers—rare

Risk Factors

Since the cause of FUO is unclear, there are no specific factors that increase your chance of this fever.


A fever is generally considered a temperature over 101 degrees Fahrenheit (38.3 degrees Celsius) but the exact number can vary. A fever of unknown origin may be consistent or occur sporadically.The fever may also be accompanied by chills, sweating, or other symptoms that are caused by the underlying illness.


You will be asked about your symptoms and medical history. A physical exam will be done. If there is no clear cause, your doctor will begin to narrow possibilities. You may be asked the following:
  • Were you traveling abroad?
  • Were you hospitalized?
  • Is your immune system damaged?
  • What medications are you currently taking?
Your bodily fluids may be tested. This can be done with:
  • Blood tests
  • Urine tests
  • Biopsy
Images may be taken of your bodily structures. This can be done with: The inside of your lungs, intestines, or sinuses may be examined. This can be done with endoscopy.
Endoscope in stomach
Copyright © Nucleus Medical Media, Inc.

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