Fever in Infants Less Than 3 Months

Etiologies and Risk Factors

Infants less than 3 months with fever have a high risk of invasive bacterial infections (8-13%) and require a comprehensive evaluation, including urine analysis, blood culture, inflammatory markers, and lumbar puncture based on risk factors, with hospitalization and empirical antibiotic therapy, according to the American College of Radiology 1, 2
Viral infections account for approximately 75% of cases of fever in infants less than 3 months, but the presence of a viral infection does not exclude the coexistence of a bacterial infection, as stated by the American College of Radiology 3, 1
The prevalence of urinary tract infections is 3-7% in febrile children 2 months to 2 years without an apparent source, according to the American Academy of Pediatrics 4

A systematic initial assessment should include urine analysis with leukocyte esterase, nitrites, leukocyte count, or Gram stain, as well as laboratory tests for inflammatory markers and blood culture, as recommended by the American College of Radiology 4, 1, 2
Lumbar puncture may be included based on clinical factors that categorize the patient as high or low risk, according to the American College of Radiology 1, 2
Chest radiography is not systematic unless there are localized respiratory signs or concern for occult pneumonia, as stated by the American College of Radiology 3, 2

Hospitalization and empirical antibiotic therapy are used in these patients, according to the American College of Radiology 1, 2
The American Academy of Pediatrics recommends that the treatment of all infections should be continued for a minimum of 48 to 72 hours beyond the time the patient becomes asymptomatic or evidence of bacterial eradication has been obtained 5, 6

Never rely solely on clinical appearance: only 58% of infants with bacteremia or bacterial meningitis appear clinically ill, as stated by the American Academy of Pediatrics 7, 4
Recent use of antipyretics can mask fever and disease severity, according to the American Academy of Pediatrics 7, 6
Do not assume that a viral infection excludes a bacterial infection: viral infections can coexist with bacterial infections, as stated by the American Academy of Pediatrics 7, 4

The American Academy of Pediatrics recommends that all well-appearing febrile newborns (≤28 days old) require immediate comprehensive evaluation including urinalysis with culture, blood culture, and lumbar puncture for CSF analysis, followed by hospitalization and empiric parenteral antibiotics until cultures are negative at 24-36 hours 8, 9, 10
Urinalysis has 94% sensitivity for UTI and 99% negative predictive value, and should be obtained by catheterization or suprapubic aspiration for both urinalysis and culture 8, 11
Blood culture should be obtained at least 1 mL of blood in a single aerobic blood culture bottle before starting antibiotics, as bacteremia occurs in 1.1-2.2% of all febrile infants and 5-10% of those with UTI 8, 11, 12

The American Academy of Pediatrics recommends ampicillin plus gentamicin as the first-line regimen for neonates (≤28 days) 10
For infants 29-60 days, ampicillin plus gentamicin or ceftriaxone/cefotaxime may be used if antibiotics are indicated 13

All neonates ≤28 days with fever require hospitalization, while infants 29-60 days who are high-risk based on clinical appearance or abnormal laboratory findings may also require hospitalization 8, 10, 13
Well-appearing infants 29-60 days with normal urinalysis, inflammatory markers, and CSF analysis may be considered for outpatient management with parenteral antimicrobial therapy and close follow-up 8, 13

Positive bacterial pathogens in urine, blood, or CSF require targeted antimicrobial therapy for duration consistent with the organism and clinical response, with a minimum of 48-72 hours beyond when the patient becomes asymptomatic or bacterial eradication is documented 8, 11, 12