What is the appropriate nitrofurantoin (Macrobid) dosing for a 10‑year‑old girl weighing 30–35 kg with an uncomplicated lower urinary‑tract infection and normal renal function?

Nitrofurantoin Dosing for a 10‑Year‑Old Girl with Uncomplicated Lower UTI

For a 10‑year‑old girl weighing 30–35 kg with uncomplicated lower urinary tract infection and normal renal function, prescribe nitrofurantoin 5–7 mg/kg/day divided into four doses (maximum 100 mg/dose) for 7 days, or at least 3 days after obtaining sterile urine. 1

Pediatric Weight‑Based Dosing

• Children under 12 years of age: The recommended dose is 5–7 mg/kg/day divided into 4 doses, with a maximum single dose of 100 mg. 1

• For a 30 kg child: This translates to approximately 150–210 mg total daily dose, divided as 37.5–52.5 mg four times daily (round to practical dosing such as 50 mg four times daily).

• For a 35 kg child: This translates to approximately 175–245 mg total daily dose, divided as 43.75–61.25 mg four times daily (round to 50 mg four times daily).

• Duration: Treat for 7 days or at least 3 days after obtaining sterile urine in pediatric patients. 1

Why Not the Adult 100 mg Twice‑Daily Regimen

• The 100 mg twice‑daily formulation (Macrobid/nitrofurantoin monohydrate/macrocrystals) is recommended only for children ≥12 years of age. 1

• Children under 12 require the macrocrystalline formulation dosed four times daily because pharmacokinetic data supporting twice‑daily extended‑release formulations are limited in younger pediatric patients.

Confirming the Diagnosis of Uncomplicated Lower UTI

• Lower‑tract symptoms only: Dysuria, frequency, urgency, or suprapubic discomfort without fever > 38 °C, flank pain, costovertebral‑angle tenderness, nausea, or vomiting. 1

• If any upper‑tract signs are present (fever, flank pain, systemic symptoms), nitrofurantoin is contraindicated because it does not achieve adequate renal tissue concentrations; switch to ciprofloxacin or a third‑generation cephalosporin. 1

Renal Function Verification

• Confirm that estimated glomerular filtration rate (eGFR) is ≥30 mL/min/1.73 m² before prescribing nitrofurantoin; the drug is contraindicated below this threshold due to insufficient urinary concentrations and increased risk of peripheral neuropathy. 1

Urine Culture Recommendations

• Routine urine culture is not required for straightforward uncomplicated cystitis in otherwise healthy children with typical symptoms. 1

• Obtain urine culture and susceptibility testing if:

  • Symptoms persist after completing therapy,
  • Symptoms recur within 2 weeks,
  • Fever or flank pain develops,
  • The patient has a history of recurrent UTIs or resistant organisms. 1

Common Pitfalls to Avoid

• Do not use nitrofurantoin for suspected pyelonephritis or any upper‑tract infection, as tissue penetration is inadequate. 1

• Do not prescribe the 100 mg twice‑daily adult formulation to children under 12 years; use the four‑times‑daily macrocrystalline preparation instead. 1

• Do not extend treatment beyond 7 days unless symptoms persist or cultures remain positive, as longer courses increase adverse‑event risk without improving efficacy. 1

• Ensure adequate hydration during treatment to prevent crystal formation. 1

Expected Efficacy and Safety

• Nitrofurantoin achieves 88–93 % clinical cure and 81–92 % bacteriological eradication in uncomplicated UTIs. 1

• The most common adverse effects are nausea and headache, with overall adverse‑event rates of 5.6–34 %; serious pulmonary or hepatic toxicity is extremely rare (0.001 % and 0.0003 %, respectively). 1, 2

• Resistance to nitrofurantoin remains < 1 % worldwide, making it an excellent first‑line choice. 1