Examkin

Question

Difficulty: MediumPharmacologic Interventions

A 62-year-old woman comes to the outpatient clinic reporting a 2-day history of dysuria and increased urinary frequency. She denies fever, chills, flank pain, nausea, or vomiting. Her medical history is significant for hypertension and a mechanical mitral valve replacement performed 4 years ago, for which she is on chronic anticoagulation. Current medications include warfarin, lisinopril, and metoprolol. Her temperature is 37.1°C (98.8°F), blood pressure is 128/82 mm Hg, and pulse is 76/min. Physical examination reveals mild suprapubic tenderness to palpation but no costovertebral angle tenderness.

Laboratory studies show:

TestResult
Serum creatinine0.9 mg/dL
INR2.6
Urinalysis
ParameterResult
ColorYellow
Specific gravity1.018
pH6.5
ProteinNegative
BloodTrace
Leukocyte esterasePositive
NitritesPositive
WBC35/hpf
RBC3/hpf

Which of the following is the most appropriate pharmacologic intervention?

  1. A
    Oral amoxicillin
  2. B
    Oral ciprofloxacin
  3. Oral nitrofurantoinAnswer
  4. D
    Oral trimethoprim-sulfamethoxazole
  5. E
    Intravenous ceftriaxone

Answer

Oral nitrofurantoin
The patient presents with acute uncomplicated cystitis. First-line empiric therapies for uncomplicated cystitis include oral nitrofurantoin, trimethoprim-sulfamethoxazole (TMP-SMX), and fosfomycin. However, this patient's medication regimen includes warfarin for a mechanical heart valve. TMP-SMX is a potent inhibitor of cytochrome P450 2C9 (CYP2C9), which is the primary enzyme responsible for metabolizing the more active S-enantiomer of warfarin. Co-administration of TMP-SMX and warfarin significantly decreases warfarin clearance, leading to a rapid, profound increase in the international normalized ratio (INR) and elevating the risk of severe hemorrhage. Therefore, oral nitrofurantoin is the most appropriate first-line pharmacologic intervention, as it provides excellent concentration in the urine, effectively covers common uropathogens (e.g., E. coli), and does not clinically interact with warfarin.

Step-by-Step Solution

1
Diagnose uncomplicated acute cystitis.
The patient has dysuria, frequency, and a supportive urinalysis (positive leukocyte esterase, nitrites, WBCs) without systemic signs of infection.
Establishing the correct diagnosis allows for the selection of appropriate empiric antimicrobial therapy.
2
Identify concurrent high-risk medications.
The patient is taking warfarin for a mechanical heart valve and has a therapeutic INR of 2.6.
Warfarin has a narrow therapeutic index and is highly susceptible to drug-drug interactions that can lead to catastrophic bleeding or thrombosis.
3
Evaluate first-line empiric antibiotic options for interactions.
First-line agents include nitrofurantoin, trimethoprim-sulfamethoxazole (TMP-SMX), and fosfomycin. TMP-SMX heavily interacts with warfarin.
TMP-SMX inhibits CYP2C9, the primary enzyme that metabolizes the active S-enantiomer of warfarin, leading to dangerously elevated INR levels.
4
Select the optimal safe pharmacologic intervention.
Oral nitrofurantoin is selected.
It provides excellent urinary concentration and uropathogen coverage without significantly interacting with warfarin.

Key Concept

In patients taking warfarin, trimethoprim-sulfamethoxazole should be avoided for the treatment of uncomplicated cystitis due to severe CYP2C9 inhibition leading to increased bleeding risk; alternative first-line agents like nitrofurantoin or fosfomycin are preferred.
Rate this question

Topics

Pharmacologic Interventions

More questions from this topic