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.