Unexplained Fever...A Difficult Diagnosis

Medical Author: Melissa Conrad Stoppler, MD
Medical Editor: William C. Shiel, Jr., MD, FACP, FACR

Usually when a person develops a fever, he or she has pain, or cough, or other symptoms that explain why the fever is occurring. But occasionally people develop fevers without an apparent reason. When fevers persist, doctors refer to such a fever as fever of unknown origin. Abbreviated FUO, this unusual form of fever is defined by the presence of fever greater than 38.3°C (101 °F) "off and on" for more than three weeks without specific cause for the fever identified.

Research has shown that in 85 to 95% of cases, the specific cause for a fever of unknown origin can eventually be identified after extensive testing, often in a hospital setting. Doctors may need to perform a variety of diagnostic tests to help them determine the exact cause of an unexplained fever. Blood tests, a thorough physical examination, and radiological studies (most commonly a chest x-ray and/or chest and abdominal CT scans) are generally performed as a first step in the investigation of unexplained fever.

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