Clinical Subject Page

Fever of Unknown Origin (FUO)

Fever of Unknown Origin (FUO) is a clinical condition characterized by persistent documented fever for which no cause is identified after an appropriate initial diagnostic evaluation. It is not a single disease but a diagnostic problem with a broad differential including infection, malignancy, inflammatory disease, and miscellaneous causes

Also called

Pyrexia of unknown origin (PUO)

ICD-10

R50.9

Specialty

Infectious

Onset

Acute

Reviewed

August 2026

On This Page

Overview

Overview
Etiology & Risk Factors

Etiology & Risk Factors
Pathophysiology

Pathophysiology
Clinical Presentation

Clinical Presentation
History Taking

History Taking
Physical Examination

Physical Examination
Investigations

Investigations
Diagnosis

Diagnosis
Management

Management
Complications

Complications
Prognosis

Prognosis
Key Points / Clinical Pearls

Key Points / Clinical Pearls

Overview

–Fever of Unknown Origin (FUO) is traditionally defined as a documented temperature of at least 38.3°C on several occasions, illness lasting more than 3 weeks, and failure to establish a diagnosis after appropriate evaluation.

-The major diagnostic categories for Fever of Unknown Origin (FUO) are:

Infection
Malignancy
Non-infectious inflammatory disease
Miscellaneous causes, including drug fever and endocrine disorders

Teaching point Fever of Unknown Origin (FUO) should be approached systematically. A detailed history and repeated physical examination often provide more useful diagnostic clues than indiscriminate laboratory testing.

Etiology & Risk Factors

Fever of Unknown Origin · Etiology & Risk Factors

Etiology — Major Categories

Category	High-yield causes	Key exam clue
Infections	Tuberculosis, infective endocarditis, deep abscesses, osteomyelitis	Most important category; think TB / endocarditis
Malignancy	Lymphoma, leukemia, renal cell carcinoma	Weight loss + night sweats + lymphadenopathy
Inflammatory / autoimmune	GCA/PMR, SLE, adult-onset Still disease, vasculitis	Systemic inflammatory features; elderly → GCA/PMR
Miscellaneous	Drug fever, thyroiditis, thromboembolic disease, factitious fever	Consider after major categories are excluded

Risk Factors

Infectious risk

• TB exposure or residence/travel in endemic areas

• Immunosuppression → opportunistic infection

• IV drug use → infective endocarditis

• Recent surgery or indwelling devices → occult infection

Other clues

• Older age → malignancy, GCA/PMR

• Autoimmune/inflammatory symptoms → rheumatologic disease

• New medication → drug fever

• Cancer history → occult/recurrent malignancy

High-Yield Exam Associations

Think infection

• TB exposure + weight loss/night sweats

• New murmur + endocarditis risk

• Localizing symptoms → deep abscess

Think noninfectious

• B symptoms/lymphadenopathy → lymphoma

• Elderly + headache/jaw claudication → GCA

• Fever after new drug → drug fever

Exam rule: FUO causes are classically grouped into infection, malignancy, inflammatory/autoimmune, and miscellaneous.

Pathophysiology

Underlying infection, malignancy, inflammatory disease, or another systemic disorder → cytokine release → hypothalamic temperature-set-point elevation → persistent fever → absence of an immediately identifiable cause

Clinical Presentation

-Symptoms:

Patients with Fever of Unknown Origin (FUO) may develop:
- Recurrent or persistent fever
- Chills or rigors
- Night sweats
- Fatigue
- Malaise
- Loss of appetite
- Unintentional weight loss
- Headache
- Myalgia or arthralgia
Additional symptoms depend on the underlying disease.

-Signs:

Documented fever
Lymphadenopathy
Hepatomegaly or splenomegaly
Cardiac murmur
Skin rash
Focal tenderness
Temporal artery abnormalities in selected older patients

-Sever Disease:

Severe Fever of Unknown Origin (FUO) may be associated with:

Severe systemic inflammation
Hemodynamic instability
Organ dysfunction
Sepsis
Progressive malignancy

Important Note Fever of Unknown Origin (FUO) is a syndrome rather than a final diagnosis, so new symptoms or examination findings should trigger reassessment of the differential diagnosis.

History Taking

-Ask about:

Fever pattern, duration, and maximum temperature
Chills, rigors, night sweats, and weight loss
Recent travel
Animal exposure
Sick contacts
Tuberculosis exposure
Dental procedures
Recent surgery or hospitalization
Current and recently started medications
Sexual history when clinically relevant
Autoimmune or inflammatory symptoms
Headache or visual symptoms
Joint or muscle pain

Physical Examination

-General Examination

Temperature
Blood pressure and heart rate
Weight
Hydration status
Skin examination
Lymph node examination

-System-Specific Examination:

Cardiovascular examination for murmurs
Respiratory examination
Abdominal examination for hepatosplenomegaly
Musculoskeletal examination
Neurological examination when indicated
Temporal artery examination in appropriate patients

Investigations

-Complete Blood Count

CBC is an important initial investigation in Fever of Unknown Origin (FUO) and may show:

Anemia
Leukocytosis or leukopenia
Neutropenia
Thrombocytopenia

-Peripheral Blood Film

Useful when hematological malignancy, hemolysis, or another blood disorder is suspected.

-Biochemistry / Specific Tests

Initial evaluation may include:

C-reactive protein (CRP) and/or erythrocyte sedimentation rate (ESR)
Renal and liver function tests
Urinalysis and urine culture
Multiple blood cultures when bacteremia or endocarditis is suspected
HIV testing when clinically appropriate
Tuberculosis testing when epidemiological risk or exposure is present
Autoimmune testing guided by clinical findings

Further testing for Fever of Unknown Origin (FUO) should be directed by the history and examination.

-Imaging

Chest X-ray
Abdominal or pelvic ultrasound when indicated
Computed Tomography (CT) of the chest, abdomen, or pelvis when clinically appropriate
Echocardiography when infective endocarditis is suspected

-Special / Confirmatory Tests

There is no single confirmatory test for Fever of Unknown Origin (FUO).

Depending on the suspected cause, further investigations may include:

Tissue biopsy
Bone marrow examination
Positron Emission Tomography/Computed Tomography (PET/CT)
Temporal artery biopsy
Specialized microbiological testing

Diagnosis

-The diagnosis of Fever of Unknown Origin (FUO) is made when persistent documented fever meets the accepted clinical definition and no cause is identified after an appropriate initial evaluation.

-The approach to Fever of Unknown Origin (FUO) is:

Confirm true fever → repeat detailed history and examination → basic laboratory and microbiological evaluation → targeted imaging → directed special testing or biopsy

Home Home Clinical Cases Infectious System Clinical Cases Fever of Unknown Origin (FUO)

Management

1. First-Line / Emergency Management

Fever of Unknown Origin (FUO) itself has no specific emergency treatment.

Urgent management is required when the patient develops:

Hemodynamic instability
Severe sepsis
Neutropenic fever
Severe immunosuppression
Organ dysfunction

2. Definitive Treatment

Treatment should target the underlying cause once identified.

Examples include:

Antimicrobial therapy for confirmed infection
Anti-inflammatory or immunosuppressive therapy for selected inflammatory diseases
Cancer-directed treatment for malignancy

3. Medical Treatment

Empirical therapy should generally be avoided in clinically stable patients because it may:

Mask diagnostic features
Reduce microbiological culture yield
Cause unnecessary adverse effects

Empirical treatment may be appropriate in selected unstable, neutropenic, or severely immunocompromised patients.

4. Surgical / Procedural Treatment

No routine surgical treatment.

Procedures may be required to diagnose or treat the underlying cause, such as:

Drainage of an occult abscess
Biopsy of a suspicious lesion
Temporal artery biopsy
Surgical treatment of an identified infectious source

5. Supportive Management

Adequate hydration
Nutritional support
Antipyretics when appropriate
Regular monitoring of vital signs

Complications

Dehydration
Electrolyte abnormalities
Weight loss
Malnutrition
Sepsis
Organ dysfunction
Complications of underlying infection
Complications of underlying malignancy or inflammatory disease

Prognosis

The prognosis of Fever of Unknown Origin (FUO) depends mainly on the underlying cause. Many patients eventually receive a specific diagnosis and respond to appropriate treatment, while some remain undiagnosed and improve spontaneously

Key Points / Clinical Pearls

Fever of Unknown Origin (FUO) is a diagnostic syndrome rather than a specific disease.
Persistent documented fever is the defining feature.
Major causes of Fever of Unknown Origin (FUO) include infection, malignancy, and inflammatory disease.
Tuberculosis, endocarditis, and occult abscesses are important infectious causes.
Lymphoma and leukemia are important malignant causes.
Drug fever and inflammatory diseases should also be considered.
A detailed travel, exposure, and medication history is essential.
Repeated physical examination may reveal new diagnostic clues.
CBC, inflammatory markers, cultures, and urinalysis are important initial tests.
Imaging should be guided by clinical findings.
There is no single confirmatory test for Fever of Unknown Origin (FUO).
Tissue biopsy may be required when a specific lesion is suspected.
Stable patients generally should not receive prolonged empirical treatment

Wright WF, Auwaerter PG. Fever and Fever of Unknown Origin: Review, Recent Advances, and Lingering Dogma. Open Forum Infect Dis. 2020;7(5):ofaa132. PubMed .
Cunha BA. Fever of Unknown Origin: Focused Diagnostic Approach Based on Clinical Clues from History, Physical Examination, and Laboratory Tests. Infect Dis Clin North Am. 2007;21(4):867-915. PubMed .
Bleeker-Rovers CP, Vos FJ, de Kleijn EMHA, et al. A Prospective Multicenter Study on Fever of Unknown Origin: The Yield of a Structured Diagnostic Protocol. Medicine (Baltimore). 2007;86(1):26-38. PubMed .
National Library of Medicine (NIH). Fever of Unknown Origin . StatPearls.