What is the correct dose of nitrofurantoin (antibiotic) for a urinary tract infection (UTI) caused by Enterococcus faecalis with a minimum inhibitory concentration (MIC) of <=16?

Nitrofurantoin Dosing for UTI Caused by Enterococcus faecalis

For uncomplicated urinary tract infections caused by Enterococcus faecalis with susceptibility ≤16, nitrofurantoin should be dosed at 100 mg PO every 6 hours. 1

Recommended Dosing Regimen for Enterococcus UTIs

• For uncomplicated UTIs due to vancomycin-resistant enterococci (VRE), nitrofurantoin 100 mg PO every 6 hours is recommended (Weak recommendation, very low quality of evidence) 1
• This dosing regimen is also appropriate for susceptible Enterococcus faecalis with MIC ≤16 2
• For standard uncomplicated UTIs not specifically caused by enterococci, the more common dosing is nitrofurantoin monohydrate/macrocrystals 100 mg twice daily for 5 days 3

Efficacy Against Enterococcus faecalis

• Nitrofurantoin demonstrates excellent in vitro activity against enterococci, with 88% of isolates showing susceptibility 4
• No enterococci isolates, including vancomycin-resistant strains, have shown resistance to nitrofurantoin in some studies (MIC ≥128 μg/mL) 2
• Nitrofurantoin maintains efficacy against multidrug-resistant enterococci, including VRE strains 5
• For E. faecalis specifically, MIC50 is 8 μg/mL and MIC90 is 64 μg/mL, indicating good susceptibility 5

Important Clinical Considerations

• Nitrofurantoin is contraindicated in patients with creatinine clearance <60 mL/min due to inadequate urinary drug concentrations and increased toxicity risk 6
• For patients with renal impairment, alternative agents such as trimethoprim-sulfamethoxazole or fosfomycin should be considered 6
• Common side effects include nausea and headache, with adverse event rates ranging from 5.6-34% 3
• Nitrofurantoin should be avoided if early pyelonephritis is suspected 3

Duration of Therapy

• While the standard duration for uncomplicated UTIs is 5-7 days 3, for enterococcal UTIs specifically, treatment should be continued until clinical resolution, typically 7-10 days 1
• Clinical cure rates with nitrofurantoin range from 88-93% for uncomplicated UTIs 3

Alternative Options for Enterococcal UTIs

• If nitrofurantoin cannot be used, fosfomycin 3 g PO as a single dose is an alternative for uncomplicated UTIs due to VRE 1, 5
• High-dose ampicillin (18-30 g IV daily in divided doses) or amoxicillin 500 mg IV or PO every 8 hours are other alternatives for uncomplicated UTIs due to VRE 1

Monitoring

• Routine post-treatment urinalysis or urine cultures are not indicated for asymptomatic patients 3
• For women whose symptoms do not resolve by the end of treatment or recur within 2 weeks, a urine culture with susceptibility testing should be performed 3