What is the recommended workup and treatment for a well child presenting with fever at 3 months old?

Workup for a 3-Month-Old Well-Appearing Infant with Fever

A 3-month-old (90-day-old) febrile infant requires a comprehensive sepsis evaluation including urinalysis with urine culture (via catheterization), complete blood count with differential, blood culture, inflammatory markers (CRP or procalcitonin), and lumbar puncture for CSF analysis and culture, followed by empiric parenteral antibiotics and hospitalization. 1, 2

Age-Specific Risk Stratification

At exactly 3 months of age (90 days), this infant sits at a critical transition point:

• Infants under 28 days have an 8-13% incidence of serious bacterial infections (SBI), with approximately 0.5% risk of bacterial meningitis 1
• The 29-90 day age group requires careful risk stratification, as approximately 58% of infants with bacteremia or bacterial meningitis appear clinically well 1
• Clinical appearance alone is unreliable for excluding serious infection in this age group 1, 2

Mandatory Laboratory Testing

Urinalysis and Urine Culture

• Obtain urine via catheterization or suprapubic aspiration—never use collection bags as they dramatically increase false-positive results 1, 2
• Urinary tract infection is the most common SBI in febrile infants, occurring in 8-13% of young febrile infants 2
• Both urinalysis and urine culture are required to confirm true UTI versus asymptomatic bacteriuria 2

Blood Work

• Complete blood count with differential to assess WBC count and absolute neutrophil count, which correlate with SBI risk 1, 2
• Blood culture must be obtained before initiating antibiotics 1, 2
• Inflammatory markers (CRP or procalcitonin) should be measured as part of initial evaluation to help risk-stratify patients 1, 2

Lumbar Puncture

• For infants under 28 days, lumbar puncture is mandatory regardless of clinical appearance 1, 2
• For infants 29-90 days (which includes your 3-month-old), the American Academy of Pediatrics recommends obtaining CSF if the infant does not meet low-risk criteria 3, 2
• Never delay lumbar puncture based on "low-risk" clinical criteria in infants under 28 days, though at exactly 90 days, risk stratification tools may be applied 1

Chest Radiograph

• Obtain chest radiograph only if respiratory signs or symptoms are present (retractions, grunting, nasal flaring, crackles, decreased breath sounds) 1, 2
• The prevalence of pneumonia in febrile infants up to 3 months is low (1-3%), so routine chest x-ray is not indicated without respiratory findings 1, 2
• Tachypnea alone is insufficient indication for chest radiograph 4

Immediate Management

Antibiotic Therapy

• Empiric parenteral antibiotic therapy must be initiated immediately after all cultures are obtained 1, 2
• Never administer antibiotics before obtaining cultures, as this will compromise diagnostic accuracy 1, 2
• The current prevalence of occult bacteremia in febrile children aged 3-36 months is 1.5-2%, with Streptococcus pneumoniae accounting for 82.9-91.9% of cases 3

Disposition

• Hospitalization is mandatory for infants under 28 days 1
• For infants 29-90 days who meet low-risk criteria, outpatient management may be considered only if close follow-up is absolutely ensured 5
• After completing the full sepsis workup, admit the infant for hospital observation pending culture results 2

Critical Pitfalls to Avoid

• Never rely on normal WBC count to rule out bacterial infection, especially in neonates 2
• Never fail to obtain cultures before starting antibiotics 1, 2
• Never use bag urine specimens—they are unreliable for UTI diagnosis 2
• Never assume well-appearing infants cannot have serious infections—approximately 5-20% of patients with occult bacteremia will develop significant sequelae (pneumonia, cellulitis, septic arthritis, osteomyelitis, meningitis, sepsis) 3
• Never perform routine chest x-rays without specific respiratory indications, as this increases unnecessary radiation exposure 4

Evidence-Based Context

The management approach has evolved significantly since the introduction of Haemophilus influenzae type b and pneumococcal conjugate vaccines, which have dramatically reduced the incidence of occult bacteremia. However, approximately 0.3% of previously well children aged 3-36 months with fever without source will still develop significant sequelae, with 0.03% developing sepsis or meningitis 3. Given these stakes and the unreliability of clinical appearance alone, an aggressive diagnostic approach remains warranted at 3 months of age.