What is the most appropriate antibiotic for a 78-year-old Caucasian female with uncomplicated cystitis who is allergic to penicillin and erythromycin?

Recommended Antibiotic for 78-Year-Old Female with UTI and Penicillin/Erythromycin Allergies

Nitrofurantoin monohydrate/macrocrystals 100 mg twice daily for 5 days is the best choice for this patient, provided her renal function is adequate (eGFR ≥30 mL/min). 1, 2

First-Line Treatment Selection

Nitrofurantoin is the optimal first-line agent because:

• It achieves clinical cure rates of 88-93% and bacterial cure rates of 81-92% 1
• It has minimal resistance patterns among uropathogens 1
• It causes limited collateral damage to normal flora 1
• Neither penicillin nor erythromycin allergies contraindicate its use 2, 3

The standard regimen is 100 mg orally twice daily for 5 days 1, 2, 3. A recent high-quality randomized trial demonstrated that 5-day nitrofurantoin was superior to single-dose fosfomycin, with 70% clinical resolution versus 58% (P = 0.004) 4.

Critical Renal Function Assessment

Before prescribing nitrofurantoin, you must assess renal function:

• If eGFR ≥60 mL/min: Nitrofurantoin is highly effective with 14.6% clinical failure rates 5
• If eGFR 30-60 mL/min: Nitrofurantoin remains effective in most cases, though failure rates increase slightly to approximately 23% 6, 5
• If eGFR <30 mL/min: Switch to fosfomycin as nitrofurantoin efficacy drops significantly 6, 5

Alternative First-Line Option if Nitrofurantoin Cannot Be Used

Fosfomycin trometamol 3 grams as a single oral dose is the preferred alternative when nitrofurantoin is contraindicated 2, 3, 7:

• Clinical cure rates of 90-91% 1, 2
• Microbiologic cure rates of 78-80% (lower than nitrofurantoin) 1, 2
• Particularly useful if eGFR <60 mL/min, where it outperforms nitrofurantoin with only 16% failure versus 23% 5
• Single-dose convenience improves adherence 2

Why Trimethoprim-Sulfamethoxazole Should Be Avoided

Do not use trimethoprim-sulfamethoxazole empirically unless you have confirmed local E. coli resistance is <20% 1, 2, 8:

• When organisms are susceptible, cure rates are 90-100% 8
• When organisms are resistant, cure rates plummet to 41-54% 8, 9
• The patient's sulfa allergy status is unclear from the question, but even without allergy, empiric use is problematic without resistance data 2, 8

Reserve Fluoroquinolones as Last Resort

Fluoroquinolones (ciprofloxacin 250 mg twice daily for 3 days) should only be used if both nitrofurantoin and fosfomycin are unsuitable 2, 3:

• They achieve 93-97% bacteriologic eradication rates 2
• However, they cause significant collateral damage and promote resistance 1, 2
• They should be preserved for more serious infections like pyelonephritis 1, 2

Agents to Absolutely Avoid

Never use amoxicillin or ampicillin empirically despite the patient having penicillin allergy (making this moot), as these agents have poor efficacy with >30% worldwide resistance rates 2, 8

Common Pitfalls to Avoid

• Prescribing nitrofurantoin without checking renal function: This is the most critical error, as efficacy drops dramatically with eGFR <30 mL/min 6, 5
• Using fluoroquinolones as first-line therapy: Despite high efficacy, reserve these for complicated infections 1, 2
• Assuming fosfomycin is equivalent to nitrofurantoin: While convenient, fosfomycin has lower microbiologic cure rates (78% vs 86%) 1, 4
• Forgetting to assess for signs of pyelonephritis: Check for fever, flank pain, or costovertebral angle tenderness, which would require different treatment 2, 3

Practical Treatment Algorithm

1. Confirm uncomplicated cystitis: Dysuria, frequency, urgency, or suprapubic tenderness WITHOUT fever or flank pain 2, 3
2. Check renal function (eGFR) 3, 5
3. If eGFR ≥30 mL/min: Prescribe nitrofurantoin 100 mg twice daily for 5 days 2, 3
4. If eGFR <30 mL/min: Prescribe fosfomycin 3 g single dose 7, 5
5. If both contraindicated: Consider fluoroquinolone (ciprofloxacin 250 mg twice daily for 3 days) 2