What lab work is recommended for a 6-month-old infant presenting with fever?

Laboratory Evaluation for a 6-Month-Old Infant with Fever

For a 6-month-old infant presenting with fever, urinalysis and urine culture should be obtained as the primary laboratory evaluation, as urinary tract infections are the most common serious bacterial infection in this age group. 1

Risk Assessment and Initial Evaluation

• Fever in infants is defined as a temperature ≥38.0°C (100.4°F) and requires careful evaluation due to the risk of serious bacterial infections (SBIs) 1
• The most common SBI in infants is urinary tract infection (UTI), with a prevalence of 5-7% in febrile infants younger than 24 months 1
• Risk of SBI is higher in certain populations, including uncircumcised male infants (up to 20% risk of UTI) 1
• Well-appearing infants aged 2-6 months have lower risk of serious infection than those under 3 months of age 1

Recommended Laboratory Tests

For All Febrile 6-Month-Old Infants:

• Urinalysis and urine culture (obtained via catheterization) 1
  • Positive leukocyte esterase, nitrites, leukocyte count, or Gram's stain can make preliminary diagnosis of UTI 1
  • Urine culture should be obtained when starting antibiotics for suspected UTI 1

Additional Testing Based on Clinical Presentation:

• Complete blood count (CBC) with differential 1
  • Elevated WBC count and absolute neutrophil count are associated with greater risk of SBI 1
• C-reactive protein (CRP) measurement 1, 2
  • CRP levels ≥1.87 mg/dL are associated with increased risk of SBI 2

Special Considerations

• Clinical variables associated with greater risk of SBI include:

  • Ill appearance (higher Yale Observation Scale score) 1
  • Longer duration of fever 1
  • Uncircumcised male infants 1
  • Absence of obvious source of fever (such as upper respiratory infection, otitis media, or bronchiolitis) 1

• Chest radiograph should be considered if there are clinical predictors for pneumonia 1

  • The prevalence of pneumonia in febrile infants up to 3 months is approximately 1-3% 1

• Lumbar puncture is generally not required for well-appearing 6-month-old infants with fever 1

  • This differs from management of younger infants (especially those <3 months) 1, 3

Diagnostic Algorithm

1. Obtain urinalysis and urine culture via catheterization 1
2. Measure CBC with differential and CRP 1, 2
3. Consider chest radiograph if respiratory symptoms are present 1
4. If laboratory values are concerning (elevated WBC, ANC, or CRP) or the infant appears ill, consider blood culture and broader evaluation 1, 4

Common Pitfalls to Avoid

• Relying solely on clinical appearance to rule out SBI - approximately 58% of infants with bacteremia or bacterial meningitis may appear clinically well 1
• Using urine collection bags instead of catheterization, which increases false-positive results 1
• Failing to obtain urine culture when starting antibiotics for suspected UTI 1
• Missing UTI in uncircumcised male infants, who have substantially higher rates of bacteriuria (36%) compared to circumcised male infants (1.6%) 1