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Pathophysiology
Clinical meaning
Neonates and young infants (0-90 days) have an immature immune system with deficient opsonization, complement activity, and T-cell function, making them highly susceptible to serious bacterial infections (SBI) including bacteremia, urinary tract infection, and meningitis. The prevalence of SBI in febrile infants under 90 days is 8-12%, and in neonates under 28 days it is as high as 20%. The Rochester Criteria and the newer Step-by-Step algorithm are validated clinical prediction rules that stratify febrile infants into low-risk and high-risk categories. Rochester criteria classify an infant as low-risk if previously healthy, term birth, no prior antibiotics, non-toxic appearance, WBC 5,000-15,000, band count <1,500, normal urinalysis, and <5 WBC/HPF on stool (if diarrhea present). The Step-by-Step approach adds procalcitonin (<0.5 ng/mL) and CRP (<20 mg/L) to improve sensitivity for identifying SBI. Ill-appearing infants or those under 21 days with fever require full sepsis workup including lumbar puncture, blood cultures, urine cultures, and empiric parenteral antibiotics regardless of laboratory values.
Diagnostics & workup:
- Apply Rochester Criteria: assess WBC (5,000-15,000 normal), band count (<1,500), urinalysis, and stool WBC if diarrhea present to classify low-risk vs high-risk
- Apply Step-by-Step algorithm: assess ill appearance, age stratification (<21 days vs 22-90 days), procalcitonin (<0.5 ng/mL low risk), CRP (<20 mg/L), and ANC (<10,000) sequentially
- Order blood cultures (aerobic), catheterized urine culture, and lumbar puncture with CSF analysis (cell count, protein, glucose, Gram stain, culture) for high-risk or all neonates <21 days
- Order CBC with differential, CRP, and procalcitonin as inflammatory biomarkers to guide risk stratification
- Consider chest X-ray only if respiratory symptoms are present; routine CXR is not recommended in well-appearing febrile infants
- Obtain HSV PCR and hepatic function panel if concern for neonatal herpes (vesicular lesions, seizures, elevated AST/ALT, maternal history)
Risk factors:
- Age under 28 days (highest risk for SBI, meningitis prevalence 1-2%)
- Prematurity (gestational age <37 weeks, immature immune function)
- Ill or toxic appearance on clinical examination
- Temperature 38.0 C or higher (rectal) in an infant under 90 days
- History of perinatal complications (prolonged rupture of membranes, maternal GBS colonization)
- No prior immunizations (incomplete passive immunity)
- Lack of prenatal care or unknown maternal infection status
Management
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Prescribing & monitoring
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Takeaways
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