Definition

Fever of unknown origin (FUO) is a higher body temperature with no clear cause, even though there has been at least one or two 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
  • 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.

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.

Endoscopy
Endoscope in stomach
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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 FUO.

Revision Information

  • Family Doctor—American Academy of Family Physicians

    http://familydoctor.org

  • Healthy Children—American Academy of Pediatrics

    http://www.healthychildren.org

  • Caring for Kids—Canadian Paediatric Society

    http://www.caringforkids.cps.ca

  • Health Canada

    http://www.hc-sc.gc.ca

  • Roth A, Basello G. Approach to Adult Patient with Fever of Unknown Origin. Am Fam Physician. 2003 Dec 1;68(11):2223. Available at: http://www.aafp.org/afp/2003/1201/p2223.html. Accessed September 25, 2014.

  • Fever of Unknown Origin (FUO). EBSCO DynaMed website. Available at: http://www.ebscohost.com/dynamed. Updated August 29, 2014. Accessed September 25, 2014.