Fever of Unknown Origin

Definition

Fever of unknown origin is a high body temperature with no clear cause, even though there has been at least one week of testing. The fever is present for at least three weeks.

Causes

The cause of this fever is unknown. In some people the cause may never be known.

Factors that may make it difficult to find a cause include:

• A common illness that does not have the usual symptoms
• Unusual illness, whose other symptoms appear later
• Person is unable to communicate about other symptoms such as someone in a coma
• Genetic condition that causes periodic fevers—very rare

Risk Factors

Since the cause of FUO’s is unclear, there are no specific factors that increase your chance of this fever.

Symptoms

A fever is considered a temperature over 100.9 degrees Fahrenheit (38.3 degrees Celsius). It may be consistent or occur sporadically. A fever is a common symptom related to both trivial and serious health issues.

You may or may not have other symptoms that are caused by the underlying illness.

Diagnosis

Your doctor will ask about your symptoms and medical history. A physical exam will be done. If there is no clear cause, your doctor will begin to narrow possibilities. The first step is to ask about your recent history such as:

• Were you traveling abroad?
• Were you hospitalized?
• Is your immune system damaged?
• What medicines are you currently taking?

Based on your history your doctor may then order some tests to look for possible causes. Tests may include:

• Blood, urine, and other body products testing
• Images of internal organs and structures with x-rays, CT scans, MRI scans, or ultrasound
• Biopsy of suspicious tissue
• Endoscopies to examine the inside of the lungs, intestines, or sinuses