Examkin

Question

Difficulty: EasyPharmacologic Interventions

A 62-year-old man presents to the clinic with a two-day history of burning with urination and increased urinary frequency. He reports no fever, chills, flank pain, or gross hematuria. His medical history is significant for a deep vein thrombosis six months ago, for which he takes warfarin. His international normalized ratio (INR) checked one week ago was 2.52.5. Vital signs are within normal limits. Urinalysis is positive for nitrites and leukocyte esterase. The patient has no known drug allergies and his renal function is normal. To effectively treat his infection while minimizing the risk of a severe drug-drug interaction that could elevate his INR, which of the following is the most appropriate empiric pharmacologic intervention?

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

Answer

Nitrofurantoin
The correct answer is nitrofurantoin. This patient has an uncomplicated urinary tract infection (cystitis) and is concurrently taking warfarin. The standard first-line empiric treatments for uncomplicated cystitis include nitrofurantoin, trimethoprim-sulfamethoxazole (TMP-SMX), and fosfomycin. However, TMP-SMX is a potent inhibitor of cytochrome P450 2C9 (CYP2C9), the primary enzyme responsible for the metabolism of the highly active S-enantiomer of warfarin. Co-administration of TMP-SMX and warfarin can lead to a rapid and significant increase in the INR, predisposing the patient to severe bleeding events. Nitrofurantoin does not have a clinically significant interaction with warfarin and is highly effective for uncomplicated cystitis, making it the most appropriate choice.

Step-by-Step Solution

1
Diagnose the patient's acute condition based on history and urinalysis.
The patient has acute uncomplicated cystitis (urinary frequency, dysuria, positive nitrites and leukocyte esterase, no systemic symptoms).
Establishing the diagnosis determines the appropriate spectrum of antimicrobial coverage needed.
2
Identify the patient's underlying comorbidities and current medications.
The patient is taking warfarin for a previous deep vein thrombosis and has a stable INR in the therapeutic range.
Current medications must be reviewed to prevent potentially life-threatening drug-drug interactions when initiating new therapy.
3
Select the best empiric antimicrobial that treats the infection without interacting with the patient's chronic medication.
Nitrofurantoin provides excellent coverage for uncomplicated cystitis and does not interact with warfarin. Trimethoprim-sulfamethoxazole is contraindicated due to severe CYP2C9 inhibition.
Choosing a safe first-line agent avoids iatrogenic bleeding complications.

Key Concept

When treating an uncomplicated urinary tract infection in a patient taking warfarin, nitrofurantoin is a safe first-line option, whereas trimethoprim-sulfamethoxazole should be avoided due to its potent CYP2C9 inhibition which significantly increases the INR.
Rate this question

Topics

Pharmacologic Interventions

More questions from this topic