Fever Of Unknown Origin (FUO)

Definition

Fever of unknown origin (FUO) refers to the presence of a documented elevation in body temperature for a specified time, for which a cause has not been found after basic medical evaluation. FUO is categorized as classic, hospital acquired FUO; FUO associated with low white blood cell counts (immunosuppression); and HIV-associated (AIDS-related) FUO.

Symptoms

There are many possible causes of FUO; generally though, a diagnosis can be found. The most frequent cause of FUO is still infection, though the percentage has decreased in the early 2000s. Tuberculosis remains an important cause, especially when it occurs outside the lungs. The decrease in infections as a cause of FUO is due in part to improved culture techniques that allow more precise identification of organisms and, therefore, more appropriate treatment. In addition, advances in diagnostic technologies have made it easier to identify non-infectious causes. For example, tumors and autoimmune diseases were as of 2004 easier to diagnose. An autoimmune disease is one that arises when the body's immune system attacks its own tissue as if it were foreign. This happens when the immune system does not recognize protein markers (antigens) on its own cells. In some cases, reactions to medications can also cause prolonged fever.

In about 10 percent of cases, no definite cause is found. In another 10 percent, "factitious fevers" (either self induced or no fever at all) are identified.

General constitutional symptoms tend to occur along with fever, including muscle aches and pains (myalgias), chills, and headache. Sometimes symptoms such as a rash suggest an allergic reaction.