What alternative oral antibiotic should I prescribe for a 79‑year‑old woman with an acute urinary‑tract infection who is intolerant to trimethoprim‑sulfamethoxazole (Bactrim) while awaiting urine culture results?

Alternative Oral Antibiotic for Bactrim-Intolerant 79-Year-Old Woman with UTI

Switch immediately to nitrofurantoin 100 mg orally twice daily for 5 days, which is the preferred first-line alternative when Bactrim causes intolerable side effects in older adults with normal renal function. 1

Rationale for Nitrofurantoin as First Choice

• Nitrofurantoin achieves approximately 93% clinical cure and 88% microbiological eradication in women with uncomplicated cystitis, comparable to or superior to other first-line agents. 1
• Worldwide resistance rates remain below 1%, making it highly reliable for empiric therapy. 1
• It causes minimal disruption to intestinal flora compared with fluoroquinolones or broad-spectrum agents, thereby reducing the risk of Clostridioides difficile infection—a critical consideration in a 79-year-old patient. 1
• The 5-day regimen is well-tolerated and avoids the gastrointestinal and CNS side effects your patient is experiencing with Bactrim. 1, 2

Critical Renal Function Check Before Prescribing

• You must verify that her estimated glomerular filtration rate (eGFR) is ≥30 mL/min/1.73 m² before prescribing nitrofurantoin, because adequate urinary concentrations cannot be achieved below this threshold. 1, 3
• If her eGFR is <30 mL/min/1.73 m², nitrofurantoin is contraindicated and you should proceed to the alternative option below. 1

Alternative If Nitrofurantoin Is Contraindicated

• Fosfomycin 3 g as a single oral dose is the next best choice if renal function precludes nitrofurantoin use. 1, 3
• Fosfomycin provides approximately 91% clinical cure, maintains therapeutic urinary concentrations for 24–48 hours, and has an initial-infection resistance rate of only 2.6%. 1, 3
• The single-dose regimen improves adherence and eliminates the risk of ongoing gastrointestinal side effects. 3
• Do not use fosfomycin if you suspect pyelonephritis (fever >38°C, flank pain, costovertebral angle tenderness), as it lacks sufficient tissue penetration for upper-tract infections. 1, 3

Why Not Other Agents

• Fluoroquinolones (ciprofloxacin, levofloxacin) should be reserved exclusively for culture-proven resistant organisms or documented failure of first-line therapy, because serious adverse effects—including tendon rupture, peripheral neuropathy, and CNS toxicity—outweigh benefits in uncomplicated UTI, especially in elderly patients at higher risk for these complications. 1, 2
• Beta-lactams (amoxicillin-clavulanate, cephalosporins) achieve only 89% clinical cure and 82% microbiological eradication, significantly inferior to nitrofurantoin or fosfomycin, and are associated with more rapid UTI recurrence due to disruption of protective vaginal flora. 1
• Trimethoprim alone could be considered but is less preferred than nitrofurantoin in the absence of culture data. 2, 4

When to Obtain Urine Culture

• Obtain urine culture and susceptibility testing now because your patient has already experienced treatment intolerance, which qualifies as an atypical presentation requiring microbiologic confirmation. 1, 2
• Culture results will guide any necessary adjustment if symptoms persist after completing the nitrofurantoin or fosfomycin course. 1
• Also obtain culture if symptoms do not resolve within 2–3 days of starting the new antibiotic, if they recur within 2 weeks, or if she develops fever, flank pain, or systemic signs. 1, 2

Management Algorithm for This Patient

1. Verify eGFR immediately. If ≥30 mL/min/1.73 m² → prescribe nitrofurantoin 100 mg PO BID for 5 days. 1, 3
2. If eGFR <30 mL/min/1.73 m² → prescribe fosfomycin 3 g single dose. 1, 3
3. Obtain urine culture and susceptibility testing today to confirm the pathogen and guide any necessary adjustment. 1, 2
4. Reassess symptoms in 2–3 days. If no improvement or worsening → adjust therapy based on culture results, reserving fluoroquinolones only for documented resistance. 1
5. If fever, flank pain, or CVA tenderness develop → suspect pyelonephritis and switch to a fluoroquinolone (ciprofloxacin 500 mg PO BID for 7 days or levofloxacin 750 mg PO daily for 5 days) or consider parenteral therapy. 1, 5

Key Pitfalls to Avoid

• Do not prescribe nitrofurantoin without confirming eGFR ≥30 mL/min/1.73 m²; this is the most common prescribing error in elderly patients. 1, 3
• Do not use fluoroquinolones empirically in a 79-year-old woman with uncomplicated cystitis; the risk of serious adverse effects is unacceptably high relative to benefit. 1, 2
• Do not use fosfomycin if upper-tract involvement is suspected; it is effective only for lower UTI. 1, 3
• Do not continue Bactrim despite intolerance; nausea and dizziness are common adverse effects that will not resolve with continued use and may worsen. 1