What is the recommended oral antibiotic for a 3-month-old patient with cough and leukocytosis (elevated White Blood Cell count) and thrombocytosis (elevated platelet count)?

Recommended Oral Antibiotic for a 3-Month-Old with Cough and Leukocytosis

For a 3-month-old infant with cough and leukocytosis suggestive of pneumonia, amoxicillin 30 mg/kg/day divided every 12 hours is the recommended oral antibiotic. 1

Critical Age-Specific Dosing Limitation

• Infants under 3 months (12 weeks) have a maximum recommended dose of 30 mg/kg/day divided every 12 hours due to incompletely developed renal function affecting amoxicillin elimination. 1
• This is significantly lower than the standard 90 mg/kg/day used in older children with pneumonia, reflecting the unique pharmacokinetic considerations in this age group. 2, 1
• The FDA explicitly states this upper dose limit for pediatric patients aged less than 12 weeks. 1

Clinical Context and Pathogen Coverage

• Leukocytosis and thrombocytosis in a 3-month-old with cough most commonly indicate a reactive process to infection, with secondary thrombocytosis occurring in 3-13% of hospitalized children. 3
• The most likely bacterial pathogens in this age group include Streptococcus pneumoniae, Haemophilus influenzae, and group B streptococci. 2, 4
• Amoxicillin provides excellent coverage for Streptococcus pneumoniae, the most common bacterial cause of pneumonia in young children. 2

Treatment Duration and Monitoring

• Treatment should continue for a minimum of 48-72 hours beyond symptom resolution, with a typical course of 10 days for respiratory tract infections. 1
• The infant should demonstrate clinical improvement within 48-72 hours; failure to improve necessitates immediate reevaluation and consideration of hospitalization. 2, 5
• If the infant fails outpatient oral therapy or presents with severe illness, intravenous ampicillin 150-200 mg/kg/day or ampicillin-sulbactam is indicated for hospitalized management. 2, 4

Important Caveats for This Age Group

• If the infant is not fully immunized against Haemophilus influenzae type b, consider amoxicillin-clavulanate instead to cover β-lactamase-producing organisms. 2
• Oral antibiotics in neonates and young infants achieve adequate serum levels but have lower bioavailability compared to parenteral administration. 6
• Any signs of severe illness (respiratory distress, poor feeding, lethargy, high fever) warrant immediate hospitalization and intravenous antibiotics rather than oral therapy. 2, 4
• The presence of intercostal or subcostal recession, severe fever, or vomiting significantly increases the risk of hospital admission and may indicate the need for parenteral therapy. 7

Administration Guidance

• Amoxicillin should be administered at the start of a meal to minimize gastrointestinal intolerance. 1
• For a typical 3-month-old weighing approximately 5.5 kg, the dose would be 165 mg divided into two doses (82.5 mg every 12 hours). 5