What is the recommended dose of nitrofurantoin for a 13-year-old patient weighing 36.4kg with a presumed urinary tract infection (UTI) and normal renal function?

Nitrofurantoin Dosing for a 13-Year-Old, 36.4kg Patient

For this 13-year-old patient weighing 36.4kg with a presumed uncomplicated UTI and normal renal function, nitrofurantoin should be dosed at 5-7 mg/kg/day divided into 4 doses (approximately 45-65 mg four times daily), given for 7 days minimum. 1

Weight-Based Dosing Calculation

• Standard pediatric dosing: 5-7 mg/kg/day divided into 4 doses, with maximum 100 mg per dose 1
• For 36.4kg patient:
  • Lower range: 5 mg/kg × 36.4 kg = 182 mg/day (approximately 45 mg four times daily) 1
  • Upper range: 7 mg/kg × 36.4 kg = 254.8 mg/day (approximately 65 mg four times daily) 1
• Practical dosing: 50 mg four times daily (every 6 hours) is a reasonable middle-ground dose that maintains therapeutic urinary concentrations 1

Treatment Duration and Administration

• Minimum duration: 7 days for uncomplicated cystitis, continuing at least 3 days after obtaining sterile urine 1
• Dosing frequency: Four times daily (every 6 hours) is essential to maintain adequate urinary drug concentrations 1
• Formulation preference: Macrocrystalline or monohydrate/macrocrystalline formulations are preferred for better gastrointestinal tolerability 1

Critical Safety Considerations

• Renal function verification: Serum creatinine must be normal before initiating therapy, as nitrofurantoin requires adequate urinary excretion for efficacy 1
• Indication limitation: Nitrofurantoin is approved only for uncomplicated lower UTI (cystitis); it is NOT appropriate for pyelonephritis or perinephric abscess 1
• Common side effects: Nausea and headache are generally mild 1

Important Clinical Caveats

• Age consideration: While nitrofurantoin is not formally approved for children under 12 years by some regulatory agencies, it is widely used off-label with established safety data; at 13 years, this patient falls within the approved age range 1
• Complete the course: Avoid premature discontinuation even if symptoms resolve earlier—ensure the full 7-day minimum course to prevent recurrence 1
• Alternative agents: If the patient appears toxic, cannot tolerate oral medications, or has suspected pyelonephritis, consider alternative therapy such as amoxicillin-clavulanate (20-40 mg/kg/day divided into 3 doses) or parenteral options 2, 3

Local Resistance Patterns

• Nitrofurantoin maintains excellent susceptibility (93.3%) against common urinary pathogens in pediatric patients, making it a preferred first-line agent for uncomplicated cystitis 2