What is the most appropriate next step in the workup of a 3-month-old febrile (feverish) infant with no obvious source of infection?

Management of Febrile 3-Month-Old Infant Without Source of Infection

Urinalysis is the most appropriate next step in the workup of this 3-month-old female infant with fever without an obvious source of infection. 1

Assessment of Risk and Decision Algorithm

This 3-month-old female infant presents with:

• 2-day history of fever (38.9°C/102.0°F)
• Nontoxic appearance
• Normal physical examination
• No apparent source of infection

Risk Stratification for UTI

The American Academy of Pediatrics (AAP) guidelines provide a clear approach for febrile infants with no obvious source:

1. First, assess if the infant appears toxic or ill enough to warrant immediate antimicrobial therapy

   • This infant appears nontoxic, so immediate antibiotics are not required

2. Second, assess the likelihood of urinary tract infection (UTI)

   • Female gender is a significant risk factor for UTI
   • The overall prevalence of UTI in febrile infants without an apparent source is approximately 5% 1
   • The prevalence of UTI among febrile infant girls is more than twice that of boys 1

3. For female infants, risk factors for UTI include:

   • White race
   • Age <12 months
   • Temperature ≥39°C
   • Fever ≥2 days
   • Absence of another source of infection 1

This patient has at least 3 risk factors (age <12 months, temperature ≥39°C, and fever ≥2 days), placing her at higher risk for UTI.

Recommended Diagnostic Approach

Based on the AAP guidelines, this infant is not in a low-risk group for UTI, and therefore requires urine testing 1.

The guidelines recommend two options:

1. Obtain urine specimen by catheterization for both culture and urinalysis
2. Obtain urine specimen by the most convenient means and perform urinalysis first

Since UTI is the most common serious bacterial infection in febrile infants without an obvious source, urinalysis is the most appropriate initial test 2.

Why Urinalysis Over Other Options:

• Chest radiography: Not indicated as first-line test without respiratory symptoms
• Complete blood count: Less specific for identifying the source of infection
• Erythrocyte sedimentation rate: Not recommended as first-line test for fever evaluation
• Respiratory viral culture: Not indicated without respiratory symptoms
• Urinalysis: Directly targets the most likely source of infection in this age group

Proper Specimen Collection

For accurate diagnosis, the AAP strongly recommends:

• Urine specimen should be obtained by catheterization or suprapubic aspiration
• Bag specimens have high false-positive rates (up to 85%) and should not be used for culture 2

Diagnostic Criteria for UTI

To establish the diagnosis of UTI, both of the following are required:

• Urinalysis results suggesting infection (pyuria and/or bacteriuria)
• ≥50,000 CFU/mL of a uropathogen cultured from a properly collected specimen 1

Common Pitfalls to Avoid

1. Relying on bag specimens: These have unacceptably high contamination rates
2. Treating based on urinalysis alone: Both urinalysis and culture are needed for definitive diagnosis
3. Delaying urine collection if antibiotics are needed: If the child appears ill, obtain urine before starting antibiotics
4. Missing UTI in female infants: Female gender is a significant risk factor for UTI
5. Overlooking the significance of fever duration: Fever ≥2 days increases UTI risk

Follow-up Management

If urinalysis is positive:

• Obtain urine culture by catheterization (if not already done)
• Initiate empiric antibiotics effective against common uropathogens
• Adjust therapy based on culture results
• Consider renal and bladder ultrasonography after UTI confirmation 1, 2

If urinalysis is negative:

• Clinical follow-up is appropriate
• Reevaluate if fever persists 1

In this nontoxic-appearing 3-month-old female with fever without source, urinalysis represents the most evidence-based first step in identifying the most common serious bacterial infection in this age group.