PSA Antimicrobial Prescribing 3 — Questions and Answers
Question 1: A patient receiving gentamicin for gram-negative bacteremia has a creatinine clearance of 30 mL/min. Which pharmacokinetic adjustment is most appropriate?
- Increase the dose, keep the interval the same
- Keep the dose the same, extend the dosing interval (Correct answer)
- Switch to a different aminoglycoside
- No adjustment needed as aminoglycosides are hepatically cleared
Correct answer: Keep the dose the same, extend the dosing interval
Aminoglycosides are renally eliminated; in renal impairment, extending the dosing interval (not reducing the dose) maintains adequate peak concentrations while allowing drug clearance.
Question 2: Which pharmacokinetic/pharmacodynamic parameter best predicts the efficacy of vancomycin against MRSA?
- Peak/MIC ratio
- Time above MIC
- AUC/MIC ratio (Correct answer)
- Trough concentration alone
Correct answer: AUC/MIC ratio
Current guidelines recommend targeting an AUC/MIC ratio of 400–600 for vancomycin, as this best correlates with clinical outcomes and minimizes nephrotoxicity.
Question 3: A patient on warfarin for atrial fibrillation is started on metronidazole for C. diff. What is the most important prescribing consideration?
- No interaction exists between metronidazole and warfarin
- Metronidazole inhibits CYP2C9, increasing warfarin levels and bleeding risk (Correct answer)
- Metronidazole induces CYP2C9, decreasing warfarin efficacy
- The interaction only occurs with IV metronidazole, not oral
Correct answer: Metronidazole inhibits CYP2C9, increasing warfarin levels and bleeding risk
Metronidazole inhibits CYP2C9, reducing warfarin metabolism and significantly increasing anticoagulant effect and bleeding risk; INR must be monitored closely.
Question 4: A patient with severe hepatic cirrhosis (Child-Pugh C) requires antibiotic therapy. Which agent requires the most significant dose reduction due to hepatic metabolism?
- Gentamicin
- Vancomycin
- Clindamycin (Correct answer)
- Ciprofloxacin
Correct answer: Clindamycin
Clindamycin is extensively hepatically metabolized and requires dose reduction in significant hepatic impairment, unlike renally cleared agents such as vancomycin or gentamicin.
Question 5: A pregnant woman in her first trimester develops a urinary tract infection. Which antibiotic should be avoided due to risk of fetal harm?
- Amoxicillin
- Nitrofurantoin at 36 weeks gestation
- Ciprofloxacin (Correct answer)
- Cephalexin
Correct answer: Ciprofloxacin
Fluoroquinolones (e.g., ciprofloxacin) are contraindicated in pregnancy due to evidence of fetal cartilage damage; beta-lactams and nitrofurantoin are safer alternatives.
Question 6: Which antibiotic class requires a loading dose strategy to achieve rapid therapeutic concentrations, particularly important in sepsis?
- Beta-lactams
- Macrolides
- Aminoglycosides with extended-interval dosing (Correct answer)
- Tetracyclines
Correct answer: Aminoglycosides with extended-interval dosing
Extended-interval aminoglycoside dosing uses a high loading dose to achieve optimal peak/MIC ratios quickly, which is critical for concentration-dependent killing in sepsis.
Question 7: A patient with a creatinine clearance of 15 mL/min requires antibiotic therapy for a skin infection. Which agent is safest without dose adjustment?
- Levofloxacin
- Trimethoprim-sulfamethoxazole
- Doxycycline (Correct answer)
- Nitrofurantoin
Correct answer: Doxycycline
Doxycycline is primarily eliminated via the fecal-biliary route and does not require dose adjustment in renal impairment, unlike most antibiotics.
PSA Antimicrobial Prescribing 3
A patient receiving gentamicin for gram-negative bacteremia has a creatinine clearance of 30 mL/min.
Which pharmacokinetic adjustment is most appropriate?