What diagnostic tests should be ordered for a baby with an unexplained fever?

Diagnostic Tests for Babies with Unexplained Fever

For babies with unexplained fever, diagnostic testing should include urinalysis, blood cultures, inflammatory markers, and potentially lumbar puncture based on age and risk factors, with imaging reserved for specific clinical indications.

Age-Based Approach to Diagnostic Testing

Neonates (0-28 days)

• Most aggressive evaluation required due to 10% risk of serious bacterial infection (SBI) and 10% mortality rate in this age group 1
• Required tests include:
  • Urinalysis and urine culture (catheterized specimen) 2
  • Complete blood count with differential 3
  • Blood culture (before antibiotics) 2
  • Lumbar puncture for CSF analysis and culture 2
  • Inflammatory markers (CRP, procalcitonin) 4
  • Chest radiograph 3
• Hospital admission and empiric antibiotic therapy is standard of care 5

Young Infants (29-90 days)

• Risk of SBI is approximately 5% in this age group 1
• Required tests include:
  • Urinalysis and urine culture (catheterized specimen) 2
  • Complete blood count with differential 3
  • Blood culture 2
  • Inflammatory markers (CRP, ESR, procalcitonin) 4
• Selective lumbar puncture based on risk stratification using validated criteria (Rochester, Philadelphia) 6
• Consider chest radiograph if respiratory symptoms present or inflammatory markers elevated 2

Older Infants (3-36 months)

• Risk of SBI drops to 0.5-1% in this age group 1
• Testing should be guided by clinical assessment:
  • Urinalysis and urine culture (especially in girls <2 years and uncircumcised boys <1 year) 6
  • Complete blood count if temperature ≥39°C 6
  • Blood culture if temperature ≥39.5°C and WBC ≥15,000/mm³ 6
  • Inflammatory markers if moderate to high suspicion of bacterial infection 4

Special Considerations for Specific Situations

Urinary Tract Infection Evaluation

• Urinary tract infections are the most common SBI in febrile infants, occurring in:
  • 8-13% of young febrile infants 2
  • 8-9% of girls <2 years 6
  • 3-4% of boys <1 year (higher in uncircumcised) 6
• Catheterized or suprapubic specimen required - bag specimens are unreliable 2
• Both urinalysis AND urine culture needed to confirm true UTI versus asymptomatic bacteriuria 2

Respiratory Infection Evaluation

• Prevalence of occult pneumonia is low (1-3%) in febrile infants <3 months 2
• Chest radiograph indicated for:
  • Any respiratory symptoms or signs 3
  • Unexplained fever with elevated inflammatory markers 2
  • Infants <28 days with fever 3

Meningitis Evaluation

• Lumbar puncture recommendations:
  • Mandatory in all febrile neonates (0-28 days) 2
  • Selective in infants 29-90 days based on risk stratification 6
  • Consider in any infant with concerning clinical features (irritability, lethargy, bulging fontanelle) 5

Warning Signs Requiring More Extensive Evaluation

• Clinical signs associated with serious bacterial infection:
  • Cyanosis 1
  • Poor peripheral circulation 1
  • Petechial rash 1
  • Rectal temperature >40°C 1
  • Inconsolability 5
  • Parental or physician concern (validated warning signs) 5

Diagnostic Testing for Prolonged Fever

• For fever lasting >3 weeks (fever of unknown origin):
  • More extensive laboratory evaluation including liver function tests 4
  • Consider serologic testing for specific infections 4
  • Consider imaging studies including PET/CT scan if initial evaluation is unrevealing 3
  • Consider evaluation for rheumatologic disorders and malignancy 7

Common Pitfalls to Avoid

• Obtaining bag urine specimens - unreliable for diagnosis of UTI 2
• Administering antibiotics before cultures - may obscure diagnosis 2
• Overlooking UTI in febrile infants without localizing signs 6
• Assuming normal WBC count rules out bacterial infection - especially in neonates 4
• Overreliance on clinical appearance in neonates - they may appear well despite serious infection 1
• Unnecessary imaging studies without specific clinical indications 2