Fever of unknown origin

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Fever of unknown origin (FUO) is a condition where a person experiences a persistent high temperature of at least 101°F (38.3°C) for more than three weeks or recurrently without any identifiable cause after diagnostic testing. While the source of the fever may not initially be identified, diagnosis is a crucial first step toward proper treatment. Early diagnosis and treatment are essential for preventing any potential complications associated with FUO.

Types of Fever of Unknown Origin (FUO)

The FUO may be divided into four categories:

Classic – People who were previously healthy are affected by classic FUO. It is characterized as a three-week-long, inexplicable fever. Classic FUO may be brought on by an infection or neoplasms like leukaemia. 

Immune-deficient – People with weakened immune systems are more susceptible to immune-deficient FUO. Chemotherapy treatment generally results in a weakened immune system.

HIV-associated – Fever can be brought on by HIV. A person with HIV is more prone to infections that might result in fevers.

Nosocomial – It appears that hospitalization causes a fever in those with nosocomial FUO. After being hospitalized for a condition other than fever, they start to experience the undiagnosed fever. 

Causes of FUO

A doctor at Best Pediatric Hospital in Hyderabad can identify the etiology of FUO by classifying the condition. The following causes of FUO can all be put into one category:

  • Infection
  • Inflammation
  • Malignancy like lymphoma, leukemia, pancreatic carcinoma etc.
  • Hepatitis, hyperthyroidism, drug use or addiction, and other conditions 

Several clinical tests are administered to a patient with an FUO to determine the FUO’s categorization. A disease that might otherwise go untreated may come to light after the FUO diagnosis.


FUO may be accompanied by other symptoms that might assist medical professionals in identifying the underlying reason. A fever typically manifests as:

  • A  temperature that is higher than 99.5°F (37.5°C) for children and adults or 100.4°F (38°C) for infants
  • Sweating
  • Chills
  • Headaches
  • Sinus congestion
  • Body or joint pain
  • Weakness
  • Fatigue
  • Sore throat
  • Cough
  • Skin rashes

How Is FUO Treated?

In up to 50% of instances, patients with FUO are released from the hospital without a definitive diagnosis. A lot of the time, FUO resolves itself in these situations. The course of care for a FUO depends on the underlying causes.

  • Infections are usually treated with antibiotics.
  • Antihistamines and Nonsteroidal Anti-inflammatory Medications (NSAIDs) may also be used to treat FUOs with no obvious underlying reasons.
  • Antiviral medication is the primary form of treatment for individuals with HIV-related fevers. Any accompanying symptoms or any consequences will then be treated.

Dos and Don’ts in managing FUO

  • It’s important to note that doctors at Best Maternity Hospitals in Kukatpally usually do not prescribe medication until a diagnosis is established for fever of unknown origin (FUO). If the cause is not identified, the doctor may continue to monitor for other symptoms.
  • If you are being assessed for FUO, do mention any new symptoms.
  • If you have a persistent fever of 101° F or above, CALL your doctor.
  • Do not go for self-medication. Follow the advice of your doctor.
  • FUO might be difficult to diagnose. Don’t lose patience.

Why choose Giggles?

Giggles Hospitals, Best Women and Children Hospital in Kukatpally best,  are committed to providing high-quality healthcare that meets the unique needs of each patient. Our team of qualified and skilled specialists offer comprehensive diagnosis, medication and immunization recommendations, and personalized consultations to prepare you for your recovery journey. Our experienced doctors ensure a comfortable and stress-free treatment experience, while also prioritizing patient privacy.

Our Doctors

Dr. Voleti Murali Krishna

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Dr. Sudheer Kumar

Consultant Paediatric Intensivist and Paediatric Cardiologist View Profile
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Dr. K. Radha Krishna

Chief Consultant Pediatrics View Profile
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Dr. B Rajsekhar

Senior Consultant Paediatrician View Profile
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Dr. M Srinivasa Rao

Consultant Paediatric Surgeon View Profile
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Dr. KV Raja Ramesh

Consultant Pediatrician View Profile
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Dr. Rentala Naveen

Paediatric Intensivist and Neonatologist View Profile
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Dr. K Raja Subba Reddy

Consultant Neonatologist & Paediatrician View Profile
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Frequently Asked Questions

A fever of unknown origin (FUO) is defined as one that is at least 101°F (38.3°C), lasts more than three weeks, or happens often without cause.

Invasive testing for fever of unknown origin (FUO) often involves biopsies of lymph nodes, liver, bone marrow, epididymal nodule, and temporal artery. These tests are typically performed only when initial testing or clinical presentation warrants further histological investigation.

Prognosis of fever of unknown origin (FUO) is influenced by the underlying illness causing the fever. Older individuals and those with cancer diagnoses have a poorer prognosis. However, children tend to have a better outcome compared to adults, without any clear explanation.

The ultimate diagnosis will determine the course of treatment for FUO, however antipyretic medications may be provided if no cause is identified. In the absence of a diagnosis, especially at an early stage, corticosteroids should be avoided.