Does Macrobid (nitrofurantoin) require renal dose adjustment in a patient with impaired renal function, specifically an estimated Glomerular Filtration Rate (eGFR) of 47?

Nitrofurantoin Use with eGFR of 47 mL/min

Nitrofurantoin can be safely used in patients with an eGFR of 47 mL/min, as this value falls above the current recommended threshold of 45 mL/min for safe use. 1

Renal Adjustment Guidelines for Nitrofurantoin

• Nitrofurantoin should be avoided and discontinued in patients with an eGFR persistently below 45 mL/min/1.73 m² 1
• For patients with an eGFR between 45-59 mL/min/1.73 m², no dose adjustment is required 1, 2
• The contraindication threshold was previously set at 40 mL/min in older product information (1988 Macrodantin), but was later changed to 60 mL/min in 2003 Macrobid product information 3

Evidence Supporting Use in Moderate Renal Impairment

• A population-based study showed that nitrofurantoin was not associated with increased risk of adverse outcomes in patients with eGFR <60 mL/min/1.73 m² compared to other antibiotics 2
• The same study demonstrated that nitrofurantoin was associated with lower odds of hospitalization for acute kidney injury compared to trimethoprim in patients with reduced renal function 2
• A review of evidence found that data supporting the contraindication of nitrofurantoin for patients with CrCl less than 60 mL/min are lacking, and limited data would support using this drug in patients with a CrCl of 40 mL/min or higher 3

Clinical Considerations

• The primary concern with nitrofurantoin use in renal impairment is reduced drug concentration in urine, potentially leading to decreased efficacy 4
• However, clinical evidence does not support significant treatment failure at eGFR levels between 45-60 mL/min/1.73 m² 4, 2
• Patients with eGFR <45 mL/min/1.73 m² have increased risk of UTI-related hospitalization and adverse outcomes, making appropriate antibiotic selection crucial 2

Monitoring Recommendations

• Regular monitoring of renal function is recommended when using nitrofurantoin in patients with borderline renal function 1
• The CKD-EPI equation is recommended for estimating eGFR in adults of any age 1
• Be vigilant for signs of treatment failure, which may indicate inadequate urinary concentrations of the drug 4

Common Pitfalls to Avoid

• Avoid prolonged treatment courses with nitrofurantoin in patients with reduced renal function, as this increases risk of adverse effects 3
• Do not rely solely on serum creatinine levels in older patients, as they may be within normal limits despite reduced renal function due to decreased muscle mass 1
• Be aware that many prescribers incorrectly dose antibiotics in CKD patients - one study found 64% of antibiotic prescriptions were dosed in excess of guidelines in patients with CKD 5

In conclusion, for a patient with an eGFR of 47 mL/min, nitrofurantoin can be used without dose adjustment as this value is above the current threshold of 45 mL/min where the drug is contraindicated.