What is the first-line antibiotic and dose for a 9-year-old child weighing 33kg with a urinary tract infection (UTI)?

First-Line Antibiotic Treatment for UTI in a 9-Year-Old Child

For a 9-year-old child weighing 33kg with a urinary tract infection (UTI), the first-line oral antibiotic treatment is cephalexin at a dose of 50-100 mg/kg per day divided into 4 doses (approximately 400-825 mg per dose). 1

Recommended First-Line Options

Oral Treatment Options (in order of preference):

• Cephalexin: 50-100 mg/kg/day divided into 4 doses 1
  • For a 33kg child: 1650-3300 mg/day total or approximately 400-825 mg per dose
• Cefixime: 8 mg/kg/day as a single daily dose 1
  • For a 33kg child: 264 mg/day
• Amoxicillin-clavulanate: 20-40 mg/kg/day divided into 3 doses 1
  • For a 33kg child: 660-1320 mg/day total or 220-440 mg per dose
• Trimethoprim-sulfamethoxazole: 6-12 mg/kg/day of trimethoprim component divided into 2 doses 1, 2
  • For a 33kg child: 198-396 mg/day of trimethoprim component or 99-198 mg per dose

Treatment Duration

• Treatment should be continued for 7-14 days 1
• Shorter courses (1-3 days) have been shown to be inferior for febrile UTIs 1

Special Considerations

When to Use Parenteral Treatment:

• If the child appears "toxic" 1
• If the child is unable to retain oral medications 1
• If compliance with oral medication is uncertain 1

Parenteral Options (if needed):

• Ceftriaxone: 75 mg/kg every 24 hours 1
• Gentamicin: 7.5 mg/kg/day divided every 8 hours 1

Important Clinical Considerations

• Local resistance patterns should guide antibiotic selection whenever possible 1
• First-generation cephalosporins like cephalexin show lower resistance rates (9.9%) compared to amoxicillin-clavulanate (20.7%) in outpatient pediatric UTIs 3
• Avoid nitrofurantoin in febrile UTIs as it does not achieve adequate serum or renal parenchymal concentrations to treat pyelonephritis 1
• Children with urinary tract abnormalities or recurrent UTIs may have higher rates of antibiotic resistance and may require broader coverage 3

Follow-up Considerations

• Adjust antibiotic choice based on urine culture and sensitivity results when available 1
• Consider renal and bladder ultrasonography for febrile UTIs to detect anatomic abnormalities 1

Common Pitfalls to Avoid

• Don't use amoxicillin alone as first-line therapy due to high resistance rates (median 75% of E. coli isolates) 1
• Don't use fluoroquinolones as first-line in children due to concerns about resistance emergence and availability of safer alternatives 1
• Don't use antibiotics that only achieve urinary concentrations (like nitrofurantoin) for treating febrile UTIs or suspected pyelonephritis 1

Remember that E. coli accounts for 80-90% of UTIs in children, and empiric therapy should target this organism while considering local resistance patterns 4.