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Fever of Unknown Origin (FUO) - Complete Reference Guide

1. Definition and Diagnostic Criteria

• Illness lasting at least 3 weeks
• Fever ≥38.3°C (≥101°F) on at least two occasions
• Diagnosis uncertain after 1 week of inpatient evaluation

The old "1 week of inpatient evaluation" requirement has been replaced by a minimum standard diagnostic workup (qualitative criteria), reflecting the reality that most evaluation is now outpatient-based.

• Harrison's Principles of Internal Medicine 22E (2025), Ch. 22

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2. Classification - The Five Etiologic Categories

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3. Epidemiology by Region (Large Studies 2005-2023)

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4. Major Causes in Detail

Infectious Causes

• Extrapulmonary disease (hepatic TB) without clear localizing features
• Miliary TB without classic chest X-ray pattern
• Pulmonary TB in immunocompromised patients (CXR may be normal in HIV)
• Key investigations: chest imaging, TST/IGRA, bronchoscopy + BAL (smear, culture, PCR), tissue biopsy

• Pre-treatment antibiotics masking typical organisms
• Atypical organisms: Bartonella, Brucella, Coxiella burnetii, Tropheryma whipplei, HACEK organisms, fungi

Non-infectious Inflammatory Causes

• Bimodal age peaks: 15-25 years and 36-46 years
• Classic triad: high quotidian fever (≥39°C) + salmon-colored evanescent rash (appears with fever, disappears when afebrile) + arthritis/arthralgias
• Markedly elevated ferritin (often >2000 ng/mL) is characteristic
• Presents as FUO in up to 10% of cases
• First-line treatment: glucocorticoids

• Autosomal recessive; Mediterranean ethnic groups (Arabs, Armenians, Turks, North Africans, Jews)
• Periodic attacks of fever + serositis (peritonitis, pleuritis, synovitis)
• First attack usually before age 10; attacks last 1-4 days
• Colchicine is the treatment of choice

Malignant Causes

• Lymphoma (Hodgkin's and non-Hodgkin's) - most common malignant cause of FUO
• Leukemia - often associated with macrocytosis
• Renal cell carcinoma - classic triad: fever + hematuria + polycythemia (paraneoplastic)
• Hepatocellular carcinoma - occurs in patients with cirrhosis
• Colorectal cancer - S. gallolyticus (formerly S. bovis) endocarditis is a strong clue
• Atrial myxoma - "tumor plop" on auscultation
• In patients with known malignancy, infection is responsible for the majority of FUO cases; the malignancy itself accounts for just under half.

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5. Diagnostic Approach

History - Key Red Flags and Clues

Initial Minimum Workup (Standard Baseline)

• CBC with differential
• Comprehensive metabolic panel (LFTs, renal function)
• ESR, CRP
• Blood cultures (x3, before antibiotics)
• Urinalysis + urine culture
• Chest X-ray
• ANA, RF (rheumatologic screen)
• HIV testing
• TST or IGRA (for TB)
• LDH, ferritin, uric acid (malignancy/Still's screening)
• Abdominal CT or ultrasound
• Echocardiography (if endocarditis suspected)

Advanced Workup

• Detects a cause in 30-60% of cases when initial workup is unrevealing
• Can be directly diagnostic (e.g., large-vessel vasculitis showing aortic wall uptake, lymphoma)
• Or identifies a metabolically active focus amenable to biopsy
• Role established by multiple studies; now the gold standard nuclear imaging modality for FUO (replaced Gallium-67)

• Lymph node (excisional > needle)
• Liver
• Bone marrow
• Temporal artery (if GCA suspected in elderly)

• A 2025-2026 emerging tool with diagnostic sensitivity of ~81.5% for infectious FUO vs. ~47% for conventional testing
• Shows promise for identifying rare or fastidious pathogens
• Should be used selectively, guided by clinical context and potential diagnostic clues (PDCs)

• IL-1β/DNA complex detection - promising for distinguishing sterile from infectious inflammation

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6. Treatment

General Principles

• Rational treatment is based on the final diagnosis
• Empirical therapy should be avoided except when the patient's condition is rapidly deteriorating
• Antibiotic or antituberculous therapy may permanently diminish the ability to culture bacteria

Empirical Therapy Options (When Indicated)

IL-1 Inhibition (Anakinra)

• Familial Mediterranean Fever (FMF)
• Cryopyrin-Associated Periodic Syndrome (CAPS)
• TNF receptor-associated periodic syndrome (TRAPS)
• Mevalonate kinase deficiency (hyper-IgD syndrome)
• Schnitzler's syndrome
• Adult-onset Still's disease

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7. Prognosis

• Patients who complete full evaluation and remain undiagnosed have a generally favorable prognosis - the large majority become symptom-free spontaneously
• FUO-related mortality risk is highest during early diagnostic phases (index admission)
• In one cohort of 168 undiagnosed FUO patients, all 4 deaths occurred during the index admission; 2 had diagnoses made only at autopsy (intravascular lymphoma, bilateral pneumonia)
• Chance of remaining undiagnosed is 2-5x higher in Europe vs. Asia

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8. Special Categories (Subgroups)

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9. Recent Guidelines and Literature (2024-2026)

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10. The Concept of "Inflammation of Unknown Origin" (IUO) - 2025 Update

• FUO and IUO share diagnostic evaluation and overlapping etiologies
• IUO is more strongly associated with NIID (less likely infectious) compared to classic FUO
• Both conditions now benefit from PET/CT and mNGS in the workup
• Undiagnosed illness (UI) is now the leading cause of FUO in Western European high-income settings

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• Harrison's Principles of Internal Medicine, 22nd Ed. (2025, McGraw Hill) - Ch. 22, Mulders-Manders & Rovers
• Goldman-Cecil Medicine International Edition - Ch. 259
• Frameworks for Internal Medicine (Lippincott) - Ch. 21
• AGIHO 2024 Guideline, Lancet Reg Health Eur [PMID 39973942]
• Antoniadou et al. 2025, Eur J Intern Med [PMID 40841245]
• Ryan K. 2024, Med Clin North Am [PMID 37951657]
• ACR Appropriateness Criteria Pediatric FUO 2024 [PMID 40409880]