What investigations are recommended for fever of unknown origin (PUO)?

PUO Investigations: A Structured Diagnostic Approach

For adults with fever of unknown origin (FUO), begin with mandatory baseline investigations including complete blood count, ESR, CRP, blood cultures, urinalysis and culture, liver enzymes, chest radiography, and abdominal/pelvic CT, followed by ¹⁸F-FDG PET/CT if the diagnosis remains elusive after initial workup. 1, 2

Definition and Diagnostic Criteria

FUO is defined as fever >38.3°C (101°F) persisting for at least 3 weeks without diagnosis despite appropriate investigation 1, 2. The modern definition emphasizes a "reasonable diagnostic evaluation" rather than strict time-based inpatient criteria, allowing for outpatient workup 3.

Initial Mandatory Investigations (First Week)

All patients require this baseline workup 2, 3:

Laboratory Tests

• Complete blood count - look for leukopenia (suggests infection/inflammatory disease), thrombocytosis (inflammatory marker)
• Inflammatory markers: ESR and CRP - elevated levels significantly predict reaching a diagnosis 3
• Blood cultures (multiple sets)
• Urinalysis and urine culture
• Liver enzymes and lactate dehydrogenase (LDH)
• Creatine kinase
• Electrolyte panel

Serologic Testing

• HIV testing (mandatory in all patients)
• Rheumatoid factor and antinuclear antibodies
• Region-specific serology: tuberculosis testing, cytomegalovirus, Epstein-Barr virus 2
• Cryoglobulins - perform early in the diagnostic process 3, 4

Imaging

• Chest radiography (mandatory baseline)
• Abdominal and pelvic ultrasonography or CT 2, 3

Critical Predictors of Reaching a Diagnosis

These findings significantly increase likelihood of diagnosis 3:

• Continuous fever pattern (vs. periodic fever)
• Fever duration <180 days
• Elevated ESR, CRP, or LDH
• Leukopenia or thrombocytosis
• Abnormal chest CT findings

Poor prognostic indicators for diagnosis (likelihood <10%): Recurrent fever pattern + normal ESR + normal hemoglobin 4

Advanced Imaging: The PET/CT Decision Point

If initial workup is unrevealing, ¹⁸F-FDG PET/CT is the next critical step 1:

Performance Characteristics

• Sensitivity: 84-86% across multiple meta-analyses 1
• Diagnostic yield: 56% (95% CI: 50-61%)
• Diagnostic accuracy: 76%
• Clinical helpfulness: 61%

Timing Considerations

• Perform PET/CT within 3 days of starting oral glucocorticoids if steroids are necessary 1
• Consider myocardial suppression preparation if cardiac etiology suspected 1
• PET/CT should guide subsequent tissue biopsy decisions 2

Invasive Procedures: When and Which

Biopsy procedures have a 42% diagnostic contribution rate 5. The choice depends on clinical findings:

Temporal Artery Biopsy

• Mandatory in patients ≥55 years old with elevated inflammatory markers 3, 4
• High yield for giant cell arteritis

Lymph Node Biopsy

• 79% yield if generalized lymphadenopathy present 4
• Zero yield if only cervical or inguinal nodes with negative chest X-ray and abdominal ultrasound 4
• Do not biopsy isolated peripheral lymphadenopathy

Bone Marrow Biopsy

• Perform in later stages if diagnosis remains elusive 3
• Zero yield if only nonspecific anemia without other specific clues 4

Liver Biopsy

• Consider in later stages 3
• Low yield with isolated abnormal liver chemistry without other findings 4

Avoid These Biopsies

• Enteric biopsy: never helpful 4

Tests to AVOID as Screening

These have zero screening value and should only be ordered with specific clinical indications 4:

• Immunologic serology panels (without specific clues)
• Endocrine investigations (without specific symptoms)
• Scintigraphic techniques (without potentially diagnostic clues)
• Echocardiography (without cardiac symptoms/signs)

Common Pitfalls

1. Misleading potentially diagnostic clues: 61 of 162 patients (38%) had only misleading clues 4. Don't chase every abnormality without clinical correlation.

2. Premature empiric therapy: Avoid empiric antibiotics or steroids - they obscure diagnosis without proven benefit 2, 6. Exception: neutropenic, immunocompromised, or critically ill patients.

3. Over-investigation without strategy: Repeating thorough history and physical examination while waiting for new clues is often more productive than ordering additional screening tests 4.

Expected Outcomes

• 51% of cases remain undiagnosed despite thorough workup 3
• Up to 75% of undiagnosed cases resolve spontaneously with good prognosis 6
• When diagnosis is reached: Infection 16-26%, inflammatory diseases 22-38%, malignancy 7-14% 3, 5

Age-Specific Modifications

For pediatric patients <3 months: Different approach required with focus on bacterial infections (urinary tract infections most common at 8-13% prevalence), with pneumonia prevalence only 1-3% 7. Rectal temperature measurement mandatory in neonates.