Fever of Unknown Origin

Written by Maureen Ash | Published on August 27, 2013
Medically Reviewed by George Krucik, MD, MBA

What Is a Fever of Unknown Origin?

A fever of unknown origin (FUO) is a fever of at least 101 degrees Fahrenheit (38.3 degrees Celsius) that either lasts for more than three weeks or occurs frequently without explanation. Even though a doctor cannot initially determine the cause of the fever, a diagnosis of FUO is a step toward finding a cure.

Types of Fever of Unknown Origin

There are four classifications of FUO: classic, nosocomial, immune-deficient, and HIV-associated.

  • Classic FUO affects previously healthy people. It is defined as an unexplained fever that lasts for three weeks. Infection or neoplasms such as leukemia may cause classic FUO. Other disorders, such as diseases that affect connective tissue, can also be the cause.
  • People with nosocomial FUO appear to acquire fever as a result of hospitalization. They are admitted for a reason other than fever and then begin to run an unexplained fever. Common causes include pulmonary embolism, septic thrombophlebitis, enterocolitis, and sinusitis.
  • Immune-deficient FUO occurs in people with compromised immune systems. This puts them at an increased risk of infection. This can occur because of chemotherapy treatment.
  • HIV itself can cause fevers. In addition, HIV makes a patient susceptible to infections that may cause fevers.

What Causes a Fever of Unknown Origin?

FUO has four primary types of causes. Recognizing the type of FUO helps a physician determine its cause. Each type of cause includes multiple conditions.

  • infections: tuberculosis, mononucleosis, Lyme disease, cat scratch fever, and others
  • inflammations: lupus, rheumatoid arthritis, inflammatory bowel disease, and others
  • malignancies: lymphoma, leukemia, pancreatic carcinoma, other cancers, and sarcomas
  • miscellaneous: fevers that are caused by drug use or abuse, are the result of hyperthyroidism or hepatitis, or don’t fit into other categories. 

A patient with a FUO will be given a variety of clinical tests to narrow down the FUO’s classification. Diagnosis of the FUO can be useful in drawing attention to an otherwise undiagnosed condition.  

What Is the Treatment for a Fever of Unknown Origin?

Treatment for a FUO varies greatly and depends on the underlying cause. In about 30 percent of cases, the patient is discharged without a definitive diagnosis. In many such cases, FUO resolves itself in time.

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Show Sources

  • Basello, G.M.,  & Roth, A.R. (2003). Approach to the Adult Patient with Fever of Unknown Origin. American Family Physician. 1;68(11), 2223-2229.
  • To, F. (2013). Fever of Unknown Origin: A Clinical Approach. University of British Columbia Medical Journal. 4(2). Retrieved August 19, 2013, from http://ojs.library.ubc.ca/index.php/ubcmj/article/view/183703

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