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Difficulty: EasyPharmacologic Interventions

During a routine telehealth visit, a 67-year-old woman reports a two-day history of dysuria, suprapubic discomfort, and feeling the urge to urinate frequently. She feels otherwise well, with no fevers, chills, or flank pain. The patient's past medical history includes paroxysmal atrial fibrillation managed with daily warfarin. Her last international normalized ratio (INR) was therapeutic at 2.5. A urine dipstick performed at a local laboratory is positive for nitrites and leukocyte esterase. Her baseline estimated glomerular filtration rate is 88 mL/min/1.73 m288 \text{ mL/min/1.73 m}^2. The local resistance rate of *Escherichia coli* to trimethoprim-sulfamethoxazole is approximately 10%10\%. To safely treat her suspected uncomplicated cystitis without disrupting her anticoagulation status, which of the following empiric regimens are most appropriate? (Select all that apply)

  1. NitrofurantoinAnswer
  2. FosfomycinAnswer
  3. C
    Trimethoprim-sulfamethoxazole
  4. D
    Ciprofloxacin
  5. E
    Amoxicillin

Answer

The correct choices are nitrofurantoin and fosfomycin, as both are first-line treatments for uncomplicated cystitis that avoid major interactions with warfarin.
The correct answers highlight therapies that are both highly effective for treating lower urinary tract infections and metabolically safe for patients on warfarin. Nitrofurantoin and fosfomycin concentrate well in the urine, effectively target common uropathogens, and do not cause meaningful cytochrome P450 inhibition, allowing the patient's INR to remain stable.

Step-by-Step Solution

1
Diagnose the acute condition.
The patient has uncomplicated acute cystitis based on symptoms (dysuria, urgency) and a positive urinalysis.
Establishing the diagnosis dictates the spectrum of empiric antimicrobial coverage needed (primarily against E. coli).
2
Identify relevant comorbid conditions and medications.
The patient is actively taking warfarin for atrial fibrillation and has normal renal function.
Warfarin has a narrow therapeutic index and is highly susceptible to cytochrome P450-mediated drug interactions.
3
Select appropriate pharmacologic therapies.
Nitrofurantoin and fosfomycin are chosen over other agents.
Both provide excellent empiric coverage for cystitis without inhibiting CYP enzymes, thereby maintaining a stable INR and avoiding bleeding complications.

Key Concept

When treating uncomplicated cystitis in a patient chronically taking warfarin, clinicians must select first-line antimicrobials like nitrofurantoin or fosfomycin that do not inhibit cytochrome P450 enzymes, avoiding dangerous elevations in the INR.
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Pharmacologic Interventions

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