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Fever of Unknown Origin

(FUO; Pyrexia of Unknown Origin)
  • Definition

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

    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
    • Illnesses with other symptoms that may appear later
    • Illnesses that may have a delayed positive test
    • Inability to communicate about other symptoms, such as an infant or someone in a coma
    • Genetic conditions that causes periodic fevers—rare
  • Risk Factors

    Since the cause of fever of unknown origin (FUO) is unclear, there are no specific factors that increase your chance of this fever.
  • Symptoms

    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.
  • Diagnosis

    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.
  • Treatment

    Treatment for the fever may not be necessary since a fever is a normal part of your immune system. Lowering the fever with medication may make it harder for your body to fight the infection, if one is present. Your doctor may advise medication to lower the fever if it is extremely high or causing other health-related problems.
    If an underlying condition is found, treatment will be based on that illness.
  • Prevention

    Since the cause is unclear, there are no steps to prevent fever of unknown origin (FUO).

    Family Doctor—American Academy of Family Physicians

    Healthy Children—American Academy of Pediatrics


    Caring for Kids—Canadian Paediatric Society

    Health Canada


    Roth A, Basello G. Approach to adult patient with fever of unknown origin. Am Fam Physician. 2003;68(11):2223.

    Fever of unknown origin (FUO) in adults. EBSCO DynaMed website. Available at: Updated August 29, 2014. Accessed September 25, 2014.

    Revision Information

    • Reviewer: David Horn, MD
    • Review Date: 08/2015
    • Update Date: 09/25/2014
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