What diagnostic tests and treatment should be initiated for a 14-day-old boy presenting with fever?

Diagnostic Workup for a 14-Day-Old Febrile Infant

A 14-day-old infant with fever requires immediate and comprehensive evaluation including urinalysis with urine culture (via catheterization), complete blood count with differential, blood culture, inflammatory markers (CRP), and lumbar puncture, followed by empiric parenteral antibiotics and hospitalization. 1, 2, 3

Why This Age Group Demands Aggressive Evaluation

• Infants under 28 days are at the highest risk for serious bacterial infections (SBIs), with an 8-13% incidence rate, warranting the most aggressive diagnostic and treatment approach. 3
• The risk of bacterial meningitis is approximately 0.5% in this age group, but the consequences of missed diagnosis include permanent neurologic sequelae and death. 1
• Approximately 58% of infants with bacteremia or bacterial meningitis may appear clinically well, making clinical appearance alone unreliable for risk stratification. 2

Essential Laboratory Tests

Urinalysis and Urine Culture (Mandatory)

• Obtain urine via catheterization or suprapubic aspiration—never use collection bags due to high false-positive rates. 1, 2
• Urinary tract infection is the most common SBI in febrile infants, with prevalence of 5-7% in infants younger than 24 months. 2
• Positive leukocyte esterase, nitrites, elevated leukocyte count, or positive Gram stain can provide preliminary diagnosis while awaiting culture results. 2

Blood Work (Mandatory)

• Complete blood count with differential to assess for elevated WBC count and absolute neutrophil count, which are associated with greater risk of SBI. 2
• Blood culture must be obtained before initiating antibiotics. 1, 3
• Inflammatory markers (C-reactive protein) should be measured as part of the initial evaluation. 1, 3

Lumbar Puncture (Mandatory in This Age Group)

• All infants under 28 days with fever require lumbar puncture to exclude bacterial meningitis, regardless of clinical appearance. 1, 3
• The decision to perform LP in slightly older infants (up to 60 days) may be based on risk stratification, but at 14 days of age, this is non-negotiable. 1
• Even when cerebrospinal fluid is normal, 10.5% of children undergoing LP have other serious bacterial infections (bacteremia 3.1%, UTI 4.1%). 4

Imaging Considerations

• Chest radiograph is indicated only if there are respiratory signs or symptoms, as the prevalence of pneumonia in febrile infants up to 3 months is low (1-3%). 1, 2
• Advanced imaging (CT, MRI, PET/CT) has no role in the initial evaluation of a febrile neonate without localizing signs. 1

Immediate Management

• Empiric parenteral antibiotic therapy must be initiated immediately after cultures are obtained, given the high risk of SBI in this age group. 1, 3
• Hospitalization is mandatory for all infants under 28 days with fever. 3
• Antibiotics should be continued until all bacterial cultures are negative at 24-36 hours and the infant is clinically well or improving. 1

Critical Pitfalls to Avoid

• Never rely on clinical appearance alone to exclude serious infection—well-appearing infants can have bacteremia or meningitis. 2
• Never use urine collection bags instead of catheterization, as this dramatically increases false-positive results. 2
• Never delay lumbar puncture in this age group based on "low-risk" clinical criteria—these criteria do not apply to infants under 28 days. 1, 3
• Never fail to obtain cultures before starting antibiotics, as this will compromise diagnostic accuracy. 1, 3