What laboratory studies should be performed in a child with fever of unknown origin (≥38.3 °C for ≥14 days) after initial history, physical exam, and basic investigations are nondiagnostic?

Laboratory Workup for Pediatric Fever of Unknown Origin

After initial basic investigations are nondiagnostic in a child with FUO (≥38.3°C for ≥14 days), proceed with a structured second-tier laboratory evaluation including: complete blood count with differential, erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), lactate dehydrogenase (LDH), liver enzymes, blood cultures, urinalysis with culture, and tuberculin skin testing, followed by targeted investigations based on any potentially diagnostic clues that emerge.

Initial Laboratory Foundation

The core laboratory panel should include 1, 2:

• Complete blood count with differential - Look specifically for leukopenia, thrombocytosis, or anemia as these predict higher likelihood of reaching a diagnosis
• ESR and CRP - Elevated inflammatory markers significantly predict finding a diagnosis; normal ESR particularly suggests lower diagnostic yield (<10% when combined with normal hemoglobin and recurrent fever pattern)
• LDH - Elevated levels are a significant predictor for reaching a diagnosis
• Liver enzymes - Nonspecific abnormalities are common but alone do not warrant invasive liver biopsy without other specific clues
• Blood cultures - Essential given 1-2% risk of invasive bacterial infection 3
• Urinalysis and urine culture - UTIs represent 8-13% of bacterial infections in young febrile children 3
• Tuberculin skin test - Part of basic screening in all age groups 4

Age-Specific Considerations

Infants ≤90 days: Higher risk population requiring more aggressive evaluation 3:

• Consider lumbar puncture based on clinical risk stratification
• Lower threshold for hospital admission and empiric antibiotics
• Infants <28 days are at highest risk for invasive bacterial infection

Older children: Can often be managed with outpatient observation after basic studies if well-appearing 4

Targeted Second-Tier Testing

Only pursue these when specific clinical clues emerge 2:

Serologic Testing

• Yersinia enterocolitica (Western blot) - Has screening value in appropriate clinical context
• Cryoglobulins - Should be checked early if vasculitis suspected
• Antinuclear antibody - In older children when autoimmune disease suspected (22% of FUO cases are noninfectious inflammatory diseases) 1

Avoid Routine Screening With:

• Broad immunologic panels - Zero screening value without specific clinical clues 2
• Endocrine investigations - Not useful as screening 2
• Microbiologic serology panels - Only order when directed by clinical findings 2

Common Diagnostic Categories

Understanding the epidemiology guides workup strategy 5, 1:

• Infections: 16-24% - Salmonellosis, tuberculosis, malaria, rickettsial diseases most common
• Autoimmune/inflammatory: 22% - Juvenile rheumatoid arthritis most frequent
• Neoplastic: 2-7% - Third most common category
• No diagnosis: 51% - Most have self-limited illness with favorable prognosis

Critical Pitfalls to Avoid

Do not give empiric antimicrobial trials - This obscures diagnosis and is contraindicated in FUO workup 5

Avoid extensive screening procedures without clinical clues 4, 2:

• Bone marrow aspiration has zero yield with only nonspecific anemia
• Liver biopsy has low yield with only nonspecific liver enzyme elevation
• Enteric biopsy was never helpful in prospective studies
• Lymph node biopsy only useful for generalized lymphadenopathy (79% yield), not isolated cervical/inguinal nodes

Do not rush to invasive testing - Serial clinical observation with repeated meticulous history and physical examination remains most efficient approach 2, 6. Approximately 50% of pediatric FUO cases resolve without specific diagnosis 6.

Predictors of Lower Diagnostic Yield

The combination of 1:

• Recurrent (not continuous) fever pattern
• Normal ESR
• Normal hemoglobin

suggests <10% likelihood of finding diagnosis - consider watchful waiting in these well-appearing children.

When to Escalate

Proceed to advanced imaging (18F-FDG PET/CT, abdominal/chest CT) only after 1, 2, 7, 8:

• Basic laboratory evaluation is complete
• Serial examinations reveal no new diagnostic clues
• Patient remains febrile beyond initial observation period
• Clinical deterioration occurs

The pace of workup should match illness severity 6 - well-appearing children can be observed with serial evaluations rather than subjected to extensive immediate testing.