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.

About one-third of fevers of unknown origin are caused by infections. Infections are also the most common cause of FUOs in children. Any type of infection, from a self-limiting common cold to HIVdisease, can result in fevers. In certain situations, a person may harbor a fever-producing infection that is not causing any recognizable physical signs or symptoms other than the fever. Microbiology techniques (culturing of body fluids from different sites to identify bacteria, viruses, fungi, or parasites) can sometimes identify an occult (hidden) infection. In other cases, blood tests that measure antibody levels in the blood can confirm whether an infection is present.