What is the definition, key diagnostic features, differential diagnosis, step‑wise evaluation, treatment considerations, recent guidelines, and key takeaways for pyrexia of unknown origin (PUO)?

Pyrexia of Unknown Origin: Comprehensive Diagnostic and Management Approach

Definition and Clinical Significance

Pyrexia of unknown origin (PUO) is defined as fever ≥38.3°C (101°F) persisting for at least 3 weeks with no diagnosis despite 3 outpatient visits or 3 inpatient days of investigation. 1 This syndrome represents a diagnostic challenge with significant implications for morbidity and mortality, as delayed diagnosis of treatable conditions—particularly infections, malignancies, and autoimmune diseases—directly impacts patient outcomes. 2

PUO is classified into four subcategories that guide investigation strategy: 1

• Classical PUO: Immunocompetent outpatients
• Nosocomial PUO: Hospitalized patients developing fever
• Neutropenic PUO: Patients with absolute neutrophil count <0.5 × 10⁹/L
• HIV-related PUO: Patients with known HIV infection

The etiology varies by subcategory and geographic location, encompassing infectious, inflammatory, malignant, and miscellaneous causes. 1

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Key Diagnostic Features and Clinical Evaluation

Critical Historical Elements

Obtain a detailed travel history with specific countries and dates visited—this is the single most critical element in narrowing the differential diagnosis, as geographic location directly predicts region-specific diseases. 3 Most tropical infections become symptomatic within 21 days of exposure. 3

Essential history components include: 2, 3, 4

• Occupational exposures: Livestock contact, unpasteurized dairy consumption (Brucella risk), healthcare work
• Animal contacts: Identify atypical bacterial infections requiring specific serologic testing
• Recent vaccinations or infectious illness: Suggests acute disseminated encephalomyelitis (ADEM), typically 1-14 days post-vaccination
• Medication history: Particularly immunosuppressants, biologics, recent antibiotics (masks diagnosis)
• Cardiac history: Previous valve disease, prosthetic material, or endocarditis

Physical Examination Red Flags

Perform repeated comprehensive physical examinations at each visit, as new findings emerge over time. 3 Specifically examine: 1, 2, 3, 4

• Cardiovascular: New regurgitant murmur (endocarditis), conduction abnormalities, pedal edema (heart failure)
• Skin: Rash, eschar (rickettsial disease), splinter hemorrhages, Janeway lesions, Osler's nodes
• Head/neck: Temporal artery tenderness (giant cell arteritis if age >50), jaw claudication
• Lymph nodes: Generalized lymphadenopathy (lymphoma, tuberculosis, HIV)
• Abdomen: Hepatosplenomegaly (malaria, lymphoma, endocarditis)
• Rectal/pelvic/breast: Occult abscesses or malignancy

Laboratory Red Flags

Obtain complete blood count with differential, inflammatory markers (CRP, ESR), and at least 3 sets of blood cultures before initiating any antimicrobial therapy. 2, 3, 4 Blood cultures have up to 80% sensitivity in typhoid and are essential for endocarditis diagnosis. 3

Critical laboratory patterns: 2, 3, 4

• Leukopenia + thrombocytopenia: Tickborne rickettsial diseases, dengue, acute HIV, typhoid, severe sepsis
• Lymphopenia: Viral infections (dengue, HIV), typhoid
• Eosinophilia (>0.45 × 10⁹/L): Parasitic or fungal infections
• ESR >40 mm/h in patient >50 years with new headache: Giant cell arteritis—requires temporal artery biopsy within 24 hours
• Severe neutropenia (ANC <0.5 × 10⁹/L): Medical emergency requiring same-day broad-spectrum antibacterials

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Differential Diagnosis by Category

Infectious Causes (Leading Category)

Tuberculosis remains the leading infectious cause globally, particularly in endemic areas, and can present with extrapulmonary manifestations including lymphadenitis. 2 Other critical infectious etiologies include:

• Endocarditis: 1, 3

  • Presents with fever, new regurgitant murmur, embolic events, or peripheral abscesses
  • Culture-negative endocarditis occurs with prior antibiotics or fastidious organisms (Brucella, Q fever, Bartonella)
  • Viridans streptococci most common after first year of life; S. aureus increasingly common in acute cases

• Atypical bacterial infections: 3

  • Brucella (unpasteurized dairy, livestock contact, Mediterranean/Middle Eastern travel)
  • Q fever (Coxiella burnetii)
  • Bartonella (cat scratch disease)
  • Require specific serologic testing

• Occult abscesses: 2, 4

  • Hepatic, splenic, psoas, renal, cerebral, vertebral
  • Require advanced imaging (CT or FDG-PET/CT) for detection

• Tropical infections in returned travelers: 3

  • Malaria (22% of febrile travelers; requires three daily blood films)
  • Dengue (5% overall, 13-18% from Asia)
  • Enteric fever (2.3%, predominantly South-East Asia)

• Opportunistic mycobacterial infections: 2

  • M. avium complex, M. kansasii in immunocompromised patients

Malignant Causes

Lymphomas and cancers of unknown primary site (CUP) account for 3-5% of all malignancies and frequently present with fever. 2 Key considerations:

• Lymphomas: 2

  • Must be excluded via immunohistochemistry in poorly differentiated cases
  • FDG-PET/CT highly sensitive for detection

• Cancers of unknown primary (CUP): 2

  • Require CT chest/abdomen/pelvis as minimal imaging standard
  • Thorough physical examination including head/neck, rectal, pelvic, breast

Autoimmune/Inflammatory Causes

Critical inflammatory conditions include: 2, 4

• Giant cell arteritis: ESR >40 mm/h, age >50, new headache, jaw claudication—requires urgent temporal artery biopsy
• Still's disease: Spiking fever, rash, arthritis, markedly elevated CRP/ESR
• Systemic juvenile idiopathic arthritis: 5% of FDG-PET/CT identified cases in children
• Inflammatory bowel disease: 5% of FDG-PET/CT identified pediatric cases

Miscellaneous Causes

• Drug fever: Consider medication-induced fever, particularly with recent drug initiation
• Factitious fever: Rare but important consideration when investigations persistently negative

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Step-by-Step Diagnostic Approach

Phase 1: Immediate Life-Threatening Exclusions (Day 1)

Rule out conditions requiring immediate intervention before proceeding with systematic workup. 2, 4

1. Severe neutropenia (ANC <0.5 × 10⁹/L) with fever: 2

   • Medical emergency requiring same-day broad-spectrum antibacterials without waiting for cultures
   • Hemodynamically unstable patients require monitoring every 2-4 hours with urgent infectious disease consultation

2. Endocarditis: 1, 4

   • Obtain urgent transthoracic echocardiography (TTE) if new murmur, embolic events, or peripheral abscesses
   • Perform ECG looking for conduction abnormalities or PR prolongation
   • Measure brain natriuretic peptide (BNP) if pedal edema present

3. Giant cell arteritis (age >50): 4

   • If ESR >40 mm/h with new headache or jaw symptoms, arrange temporal artery biopsy within 24 hours

4. Malaria in returned travelers: 2, 3

   • Obtain three daily blood films immediately
   • Consider viral hemorrhagic fever risk with appropriate isolation if epidemiologically relevant

Phase 2: Mandatory Initial Workup (Days 1-3)

Complete the following investigations before initiating empiric antimicrobial therapy: 2, 3, 4

Laboratory investigations:

• At least 3 sets of blood cultures (before any antibiotics)
• Complete blood count with differential
• CRP and ESR
• Comprehensive metabolic panel including liver function tests
• Urinalysis and urine culture
• HIV testing
• Tuberculin skin test or interferon-gamma release assay

Imaging:

• Chest radiography (first-line imaging for all patients) 3
• CT chest, abdomen, and pelvis with IV contrast (minimal imaging standard) 2, 4

Additional testing based on exposure history:

• Malaria blood films (three daily) if tropical travel 2, 3
• Brucella serology if livestock/unpasteurized dairy exposure 3
• Q fever and Bartonella serology if relevant exposures 3

Phase 3: Advanced Imaging (Days 3-7)

If initial workup unrevealing, proceed to FDG-PET/CT—this has the highest diagnostic yield in PUO. 1, 2

FDG-PET/CT performance characteristics: 1

• Sensitivity: 84-86%
• Specificity: 52-63%
• Diagnostic yield: 56%
• Diagnostic accuracy: 76%
• High clinical impact in 79% of cases (prompts specialist referrals or antimicrobial changes)
• 88% accuracy in identifying fever source in immunosuppressed patients

Critical timing considerations: 1

• Perform FDG-PET/CT within 3 days of initiating oral glucocorticoid therapy if steroids necessary
• Consider myocardial suppression preparation when potential cardiac etiology exists
• Note: Insufficient evidence for FDG-PET/CT in PUO with normal inflammatory markers (CRP, ESR)

Phase 4: Targeted Invasive Procedures (Days 7-14)

Based on FDG-PET/CT or CT findings, proceed to tissue diagnosis: 1, 2

For lung infiltrates:

• Bronchoscopy with bronchoalveolar lavage (BAL) within 24 hours of clinical indication 1, 2
• BAL at segmental bronchus supplying radiographic abnormalities
• Transbronchial biopsies contraindicated in febrile neutropenic and thrombocytopenic patients 2
• If tissue required: CT-guided percutaneous biopsy, video-assisted thoracoscopy, or open-lung biopsy 1, 2

For suspected endocarditis with negative TTE:

• Transesophageal echocardiography (TEE) has superior sensitivity and specificity 1, 3
• Repeat TTE/TEE within 7-10 days if initially negative but clinical suspicion remains high 1

For lymphadenopathy or masses:

• Excisional lymph node biopsy with immunohistochemistry (not fine needle aspiration)
• CT-guided biopsy of accessible lesions

Phase 5: Specialist Referrals

Involve specialists early based on clinical pattern: 1, 2

• Infectious disease: All PUO cases, particularly with travel history or immunosuppression
• Cardiology: Suspected endocarditis, new murmur, conduction abnormalities
• Rheumatology: Elevated inflammatory markers without infection, suspected vasculitis
• Hematology/oncology: Lymphadenopathy, cytopenias, suspected malignancy
• Surgery: Occult abscesses requiring drainage

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Treatment Considerations

General Principles

Do not initiate empiric antimicrobial therapy in stable patients until diagnostic workup is complete, as this obscures diagnosis. 3, 4 The priority is identifying the underlying cause to enable targeted therapy, which directly impacts morbidity and mortality outcomes.

Risk-Stratified Treatment Approach

High-risk patients (severe neutropenia, hemodynamic instability):

Immediate empiric antibacterials required: 2

• Severe neutropenia (ANC <0.5 × 10⁹/L): Same-day broad-spectrum antibacterials
• Antibacterial prophylaxis: Levofloxacin or ciprofloxacin 500 mg daily for severe neutropenia

48-hour reassessment algorithm: 2

• Stable, afebrile, neutrophil recovery (ANC ≥0.5 × 10⁹/L): Transition to oral antibiotics for low-risk cases
• Still febrile but clinically stable: Continue initial empiric regimen while pursuing further diagnostics
• Clinically unstable: Escalate antimicrobial coverage or rotate agents in consultation with infectious disease specialists

Empirical antifungal therapy indications: 2

• Fever persisting >4-6 days with rising CRP despite antibacterials
• Suspected invasive aspergillosis: First-line voriconazole or liposomal amphotericin B
• Refractory invasive aspergillosis: Add echinocandin to regimen

Low-risk stable patients:

Complete diagnostic workup before antibiotics: 3, 4

• Obtain all cultures and serologies first
• Consider withholding antibiotics for ≥48 hours while obtaining additional cultures in non-acutely ill patients with negative initial cultures
• Treat confirmed diagnoses according to pathogen-specific guidelines once identified

Supportive Care Measures

Provide symptomatic management while pursuing diagnosis: 2, 4

• Antipyretics for comfort (acetaminophen, NSAIDs if not contraindicated)
• Adequate hydration and nutritional support
• Monitor vital signs and clinical status closely
• Hemodynamically unstable patients: Monitoring every 2-4 hours

Targeted Therapy Based on Diagnosis

Once etiology identified, initiate pathogen/disease-specific treatment: 2, 3

• Tuberculosis: Standard four-drug regimen (isoniazid, rifampin, pyrazinamide, ethambutol)
• Endocarditis: Prolonged IV antibiotics per organism sensitivities; surgical consultation for valve complications
• Atypical bacteria: Doxycycline for Brucella, Q fever; azithromycin for Bartonella
• Malaria: Artemisinin-based combination therapy or appropriate antimalarial per species and resistance pattern
• Lymphoma: Chemotherapy per histologic subtype
• Giant cell arteritis: High-dose corticosteroids (prednisone 40-60 mg daily)
• Still's disease: NSAIDs, corticosteroids, or biologics (IL-1 or IL-6 inhibitors)

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Recent Guidelines and Evidence

EANM/SNMMI 2025 Guidelines on FDG-PET/CT

The most recent and highest-quality guideline (2025) from the European Journal of Nuclear Medicine and Molecular Imaging establishes FDG-PET/CT as the advanced imaging modality of choice for PUO after standard workup. 1

Key recommendations: 1

• Indication: Evaluation of patients with FUO/IUO without diagnosis despite standard workup
• Timing: Perform within 3 days of initiating oral glucocorticoids if steroids necessary
• Insufficient evidence: FUO with normal inflammatory markers (CRP, ESR)
• Performance: Meta-analyses of 23 studies (1,927 patients) show 84% sensitivity, 63% specificity, 56% diagnostic yield

European Heart Journal 2009 Endocarditis Guidelines

Endocarditis guidelines emphasize early echocardiography and obtaining blood cultures before antibiotics. 1

Key recommendations: 1

• TTE as first-line imaging in all suspected cases
• TEE in patients with high clinical suspicion and normal TTE
• Repeat TTE/TEE within 7-10 days if initially negative but suspicion remains
• Obtain blood cultures before any antimicrobial therapy
• Early involvement of cardiology and infectious disease specialists

ESCMID-ECMM-ERS 2018 Aspergillosis Guidelines

For immunocompromised patients with PUO and lung infiltrates, bronchoscopy with BAL should be available within 24 hours. 1

Key recommendations: 1

• High-resolution chest CT when lung infiltrates identified
• BAL at segmental bronchus supplying radiographic abnormalities
• Definitive pathogens: P. jirovecii, Gram-negative aerobes, Nocardia, M. tuberculosis, Aspergillus
• Positive galactomannan (blood or BAL) diagnostic for aspergillosis
• Urgent antimicrobial therapy should not be postponed by bronchoscopy

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Key Takeaways for Clinical Practice

Diagnostic Priorities

1. Complete blood cultures (≥3 sets) before any antibiotics—this is non-negotiable in stable patients. 2, 3, 4

2. Travel history with specific countries and dates is the single most critical historical element. 3

3. FDG-PET/CT after unrevealing initial workup has 56% diagnostic yield and 79% clinical impact. 1, 2

4. Repeated physical examinations reveal new findings over time—examine cardiovascular, skin, lymph nodes, abdomen at each visit. 3

5. Severe neutropenia (ANC <0.5 × 10⁹/L) with fever is a medical emergency requiring same-day antibacterials. 2

Critical Pitfalls to Avoid

1. Do not start empiric antibiotics in stable patients before completing diagnostic workup—this obscures diagnosis and worsens outcomes. 3, 4

2. Do not attribute fever to drug effect or assume benign self-limited illness without excluding life-threatening causes (malaria, endocarditis, giant cell arteritis). 3, 4

3. Do not diagnose "fever of unknown origin" in resource-limited settings—always assume and treat infection when fever cannot be explained. 2

4. Do not delay temporal artery biopsy beyond 24 hours in patients >50 years with ESR >40 mm/h and new headache. 4

5. Do not perform transbronchial biopsies in febrile neutropenic and thrombocytopenic patients—use alternative tissue sampling methods. 2

Prognostic Considerations

Between 1-14% of PUO cases remain undiagnosed after comprehensive evaluation, but this cohort has a good prognosis. 3, 5 Self-limiting viral infections account for 37% of undifferentiated febrile illnesses when comprehensive viral testing is performed. 3 However, the priority remains excluding treatable life-threatening conditions before accepting an undiagnosed fever as benign.