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Difficulty: MediumPharmacologic Interventions

A 64-year-old woman comes to the primary care clinic reporting two days of dysuria, urinary frequency, and suprapubic discomfort. She has no fever, flank pain, or vaginal discharge. Her medical history is significant for a mechanical mitral valve replacement, for which she takes chronic warfarin therapy. Her last international normalized ratio (INR) was 3.1 one week ago. She has a documented severe anaphylactic allergy to fluoroquinolones. Vital signs are within normal limits. Urinalysis is positive for leukocyte esterase and nitrites. The physician determines she has an uncomplicated urinary tract infection and plans to prescribe a 3-day course of oral trimethoprim-sulfamethoxazole. Which of the following is the most appropriate pharmacologic management strategy for this patient?

  1. Preemptively reduce the warfarin dose by 25% to 50% and recheck the INR in 3 to 5 daysAnswer
  2. B
    Continue the current warfarin dose and recheck the INR after completing the antibiotic course
  3. C
    Prescribe oral ciprofloxacin instead of trimethoprim-sulfamethoxazole to avoid altering warfarin metabolism
  4. D
    Administer oral vitamin K concurrently with the antibiotic to stabilize the coagulation cascade
  5. E
    Prescribe empiric oral ampicillin instead of trimethoprim-sulfamethoxazole

Answer

Preemptively reduce the warfarin dose by 25% to 50% and recheck the INR in 3 to 5 days
The correct answer accurately identifies the necessary management for a known, severe drug-drug interaction. Trimethoprim-sulfamethoxazole is a potent inhibitor of the cytochrome P450 2C9 (CYP2C9) enzyme, which is responsible for the clearance of S-warfarin (the more pharmacologically active enantiomer). Concurrent administration significantly increases serum warfarin levels and the international normalized ratio (INR), leading to a high risk of bleeding. Because the antibiotic course is short but the interaction is immediate, the standard of care is to preemptively reduce the warfarin dose by 25% to 50% upon initiation of the antibiotic, and to recheck the INR in 3 to 5 days to ensure it remains in the therapeutic range.

Step-by-Step Solution

1
Identify the primary diagnosis and intended treatment.
The patient has uncomplicated cystitis, and the physician intends to prescribe trimethoprim-sulfamethoxazole (TMP-SMX).
Establishing the clinical scenario is required before assessing pharmacologic interactions.
2
Assess potential contraindications to alternative treatments.
The patient has an anaphylactic allergy to fluoroquinolones, ruling out ciprofloxacin or levofloxacin. Ampicillin is also a poor choice due to widespread E. coli resistance.
This confirms that TMP-SMX remains the most appropriate antibiotic choice despite its potential interactions.
3
Evaluate the pharmacokinetic interaction between TMP-SMX and the patient's chronic medications.
TMP-SMX is a potent inhibitor of CYP2C9, the enzyme responsible for metabolizing the highly active S-enantiomer of warfarin.
Understanding the mechanism of the interaction is critical for predicting its clinical effect (increased INR).
4
Determine the necessary clinical intervention to mitigate the interaction.
The standard of care is to preemptively reduce the warfarin dose by 25% to 50% and closely monitor the INR within 3 to 5 days.
This maintains therapeutic anticoagulation while preventing life-threatening hemorrhage.

Key Concept

Pharmacokinetic drug interactions involving warfarin and CYP2C9 inhibitors
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Topics

Pharmacologic Interventions

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