What is a Fever of Unknown Origin?

A fever of unknown origin is a fever that onsets without a known cause and persists, usually intermittently, for more than three weeks. Commonly, extensive medical evaluation is needed to find out why a patient has developed a fever, and treatment must be delayed until the testing is over, as medications could interfere with test results. If there are worries about complications caused by high fever, drugs may be prescribed to reduce the fever, and the patient can also be given dietary recommendations, as well as being provided with cooling to bring the temperature down.

To meet the diagnostic criteria for a fever of unknown origin, a fever must be above 101 degrees Fahrenheit (38.3 degrees Celsius), appearing periodically over a three-week period with no clear cause. The fever may occur alone and can be accompanied with other symptoms like coughing. Patients may spend a week or more being evaluated for potential causes and testing can include bloodwork, medical imaging, urinalysis, and a variety of other measures to find out more about what is happening inside the patient's body.

Around a third of these cases are the result of an infection. Antibiotics should kill the organisms and resolve the fever along the way. Malignant growths can also sometimes be involved, as can connective tissue diseases, especially inflammatory conditions. In as many as 15% of cases, no cause can be found for a fever of unknown origin. This can be frustrating for patients and care providers, as treatment is difficult when the causes of a fever are not known.

There are several concerns with persistent high fever. One is the risk of damage to organs, including the brain, caused both by the fever and by the secondary increased need for oxygen. Patients may need supplemental oxygen to prevent deprivation in important tissues while being evaluated for a fever of unknown origin. The untreated root cause of the fever may also be a source for concern, as the longer is goes unidentified, the more risk of complications. A patient in crisis is also usually more vulnerable to opportunistic infections and other secondary complications, and these could endanger the patient and make diagnosis even more difficult.

People with a fever of unknown origin are usually seen on an outpatient basis first to see if the cause can be determined. If the patient develops complications or appears to be at increased risk, hospitalization may be recommended for the purpose of providing close and attentive monitoring.