Fever of Unknown Origin

By Medically reviewed by hellodoktor


What is fever of unknown origin?

Fever of unknown origin (FUO) is body temperature ≥ 38.3° C rectally that does not result from transient and self-limited illness, rapidly fatal illness, or disorders with clear-cut localizing symptoms or signs or with abnormalities on common tests such as chest x-ray, urinalysis, or blood cultures.

FUO is currently classified into 4 distinct categories:

  • Classic FUO: Fever that lasts for more than 3 week with no identified cause after 3 days of hospital evaluation or more than 3 outpatient visits
  • Health care–associated FUO: Fever in hospitalized patients receiving acute care and with no infection present or incubating at admission if the diagnosis remains uncertain after 3 days of appropriate evaluation
  • Immune-deficient FUO: Fever in patients with immunodeficiencies if the diagnosis remains uncertain after 3 days of appropriate evaluation, including negative cultures after 48 hours
  • HIV-related FUO: Fever that lasts for more than 3 weeks in outpatients with confirmed HIV infection or more than 3 days in inpatients with confirmed HIV infection if the diagnosis remains uncertain after appropriate evaluation

How common is fever of unknown origin?

Please discuss with your doctor for further information.


What are the symptoms of fever of unknown origin?

FUO may be accompanied by other symptoms that can help doctors determine the underlying cause.

Typical symptoms of a fever include:

  • A temperature that exceeds 38°C for babies or 37.5°C for children and adults
  • Sweating
  • Chills
  • Headaches

Other symptoms that typically accompany fever include:

  • Body or joint aches
  • Weakness
  • Sore throat
  • Fatigue
  • Cough
  • Rash
  • Sinus congestion

There may be some symptoms not listed above. If you have any concerns about a symptom, please consult your doctor.

When should I see my doctor?

If you have any signs or symptoms listed above or have any questions, please consult with your doctor. Everyone’s body acts differently. It is always best to discuss with your doctor what is best for your situation.


What causes fever of unknown origin?

Recognizing the type of FUO helps a physician find its cause. Causes of FUO can be categorized as any of the following:

  • Infection: tuberculosis, mononucleosis, Lyme disease, cat scratch fever, endocarditis, and others
  • Inflammation: lupus, rheumatoid arthritis, inflammatory bowel disease, and others
  • Malignancy: lymphoma, leukemia, pancreatic carcinoma, and other cancers and sarcomas
  • Miscellaneous: fevers caused by drug use or abuse,hyperthyroidism, hepatitis, and factors that don’t fit into other categories

A person with a FUO is given several clinical tests to narrow down the FUO’s classification. Diagnosis of the FUO can also draw attention to an otherwise undiagnosed condition.

Risk factors

What increases my risk for fever of unknown origin?

Please discuss with your doctor for further information.

Diagnosis & treatment

The information provided is not a substitute for any medical advice. ALWAYS consult with your doctor for more information.

How is fever of unknown origin diagnosed?

In some cases, a wait-and-see approach is often used for short-term fevers that aren’t accompanied by any red flag symptoms. Once a fever lasts long enough to be classified as a fever of unknown origin, your doctor may run some tests to determine the underlying cause.


Your doctor will likely ask if you’ve:

  • been out of the country
  • had any environmental exposures
  • had any changes in your daily environment

If you work with animals, your doctor may consider animal-borne illnesses. They’ll also ask about your family history and illnesses such as lymphoma or rheumatic fever.

Blood work and physical exam

Your doctor may also run blood tests to check for certain conditions, including autoimmune conditions that might not have many obvious symptoms. They’ll examine your skin carefully for signs of pallor, rash, or jaundice.

If blood work or the physical exam turns up any positive indicators, the doctor will order more tests before confirming a diagnosis.

Culture tests

Blood, urine, and sputum cultures may be used to check for causes such as bacteria and fungi. Special tests can also help check for atypical bacterial, fungal, or viral infections.

Imaging tests

An endocardiogram may be used to evaluate your heart if your doctor hears a murmur or strongly suspects endocarditis. This is an infection of one of the heart valves. Chest X-rays may be used to inspect the lungs.

How is fever of unknown origin treated?

Treatment for a FUO varies depending on the cause.

Nonsteroidal anti-inflammatory drugs (NSAIDs) and antihistamines may also be used to treat FUOs that have no trace of underlying causes. In many people, these medications can help reduce the fever itself.

People whose fevers are thought to have an immune-deficient origin may be treated with broad-spectrum antibiotics. These target the most likely pathogens. Infections are responsible for between 20 and 40 percent of all fevers of unknown origin.

In people with HIV-associated fevers, treatment focuses on treating HIV with antiviral drugs. After that, any associated symptoms or complications that may stem from it will be addressed.

Lifestyle changes & home remedies

What are some lifestyle changes or home remedies that can help me manage fever of unknown origin?

The following lifestyles and home remedies might help you cope with fever of unknown origin:

  • Drink plenty of fluids. Fever can cause fluid loss and dehydration, so drink water, juices or broth. For a child under age 1, use an oral rehydration solution such as Pedialyte. These solutions contain water and salts proportioned to replenish fluids and electrolytes. Pedialyte ice pops also are available.
  • You need rest to recover, and activity can raise your body temperature.
  • Stay cool. Dress in light clothing, keep the room temperature cool and sleep with only a sheet or light blanket.

If you have any questions, please consult with your doctor to better understand the best solution for you.

Hello Health Group does not provide medical advice, diagnosis or treatment.

Review Date: August 24, 2018 | Last Modified: September 13, 2019

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