Fever of Unknown Origin
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Fever of Unknown Origin

(FUO; Pyrexia of Unknown Origin)

En Español (Spanish Version)


An elevated temperature that eludes diagnosis is given the term “fever of unknown origin,” or FUO. A FUO has been defined as an intermittent temperature of at least 101°F for over three weeks with at least a week’s worth of attempts to find the cause.

Generally, the cause of a fever is obvious—usually it is an infection, and usually the site of the infection and the nature of the infecting agent are easy to figure out. When the usual investigations yield no answer, yet the fever persists, a tentative diagnosis of FUO is made until the cause can be identified.


There are many rare causes of temperature elevation. Sifting through them for the right answer challenges many a good physician. The following list is by no means exhaustive but merely includes the most common of the uncommon causes. Over 140 separate diseases are listed in Harrison’s Principles of Internal Medicine .

Rheumatoid Arthritis

Rheumatoid Arthritis

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Risk Factors

The following factors increase your chance of developing a FUO. If you have any of these risk factors, tell your doctor:

  • Foreign travel, especially to developing and tropical countries
  • Current medications (both prescription and over-the-counter)
  • Cancer or brain tumor
  • Collagen vascular disease (an autoimmune disorder of connective tissue)
  • Current or recent hospitalization
  • Similar problems in your family


If you experience any of these symptoms, do not assume it is due to a FUO. A fever is a very common indication of many problems, both serious and trivial. If you experience any one of them, see your physician.

  • Elevated temperature by thermometer reading
  • Sweats
  • Chills
  • Aching all over


Your doctor will ask about your symptoms and medical history, and perform a physical exam. You can help by taking your temperature with a thermometer several times a day. Your primary care physician may refer you to a specialist in infectious diseases or possibly oncology. The first efforts after the usual evaluation will be to narrow the possibilities by examining the circumstances under which the fever began. Were you traveling abroad? Were you hospitalized? Is your immune system damaged, for example by AIDS? What medications are you currently taking?

Many different tests may be indicated at some point in evaluating a FUO. Tests may include the following:

  • Exhaustive studies of blood, urine, and all other bodily products
  • Exhaustive imaging studies: x-rays, CT and MRI scans, ultrasound examinations
  • Nuclear medicine studies
  • Endoscopies (lungs, stomach and intestines, sinuses, etc.)
  • Biopsies (samples taken by knife or needle) of suspect tissues


Endoscope in stomach

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There is no treatment for a FUO until the underlying disease is identified. When its cause is discovered, treatment will follow.


There are so many causes for a FUO that prevention includes everything one does to stay healthy. One important item needs mentioning: be sure to take all preventive measures recommended by the public health department when you travel abroad.


Fever of Unknown Origin
University of Maryland Medical Center


BC Health Guide
British Columbia Ministry of Health

Health Canada


Amin K, Kauffman CA. Fever of unknown origin: a strategic approach to this diagnostic dilemma. Postgraduate Medicine Online website. Available at: http://www.postgradmed.com/issues/2003/09_03/amin.htm . Accessed September 14, 2005.

Beers MH, Berkow R. Biology of infectious disease. In: The Merck Manual . 17th ed. Merck & Co: West Point, PA; 1999.

Gelfand JA, Callahan MV. Fever of unknown origin. In: Kasper DL, Braunwald E, Fauci A, Hauser S, Longo D, Jameson JL. Harrison's Principles of Internal Medicine . 16th ed. New York: McGraw-Hill; 2005: 16-121.

Roth AR, Basello GM. Approach to the adult patient with fever of unknown origin. Dec 1, 2003. American Family Physician website. Available at: http://www.aafp.org/afp/20031201/2223.html . Accessed September 14, 2005.

Last reviewed January 2008 by David L. Horn, MD, FACP

Please be aware that this information is provided to supplement the care provided by your physician. It is neither intended nor implied to be a substitute for professional medical advice. CALL YOUR HEALTHCARE PROVIDER IMMEDIATELY IF YOU THINK YOU MAY HAVE A MEDICAL EMERGENCY. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with any questions you may have regarding a medical condition.

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