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Difficulty: MediumInfectious Diseases

A 62-year-old woman is receiving care on the inpatient ward for a catheter-associated bloodstream infection. She was admitted 3 days ago with high fever, chills, and localized erythema around a peripheral intravenous catheter site. The catheter was promptly removed, and she was started on empiric intravenous vancomycin and piperacillin-tazobactam. Today, she reports feeling much better. Her temperature is 37.1°C (98.8°F), blood pressure is 128/82 mmHg, pulse is 76/min, and respirations are 14/min. The erythema at the prior catheter site has significantly improved. Blood cultures obtained on admission are now finalized and grow *Staphylococcus aureus*. Susceptibility testing demonstrates resistance to penicillin G and susceptibility to oxacillin, cefazolin, vancomycin, and daptomycin. Which of the following is the most appropriate next step in pharmacotherapy?

  1. Transition to intravenous cefazolin monotherapyAnswer
  2. B
    Continue intravenous vancomycin monotherapy
  3. C
    Continue intravenous vancomycin and piperacillin-tazobactam
  4. D
    Continue intravenous piperacillin-tazobactam monotherapy
  5. E
    Transition to intravenous daptomycin monotherapy

Answer

Transition to intravenous cefazolin monotherapy
The patient has a bloodstream infection caused by methicillin-susceptible Staphylococcus aureus (MSSA), as indicated by the organism's susceptibility to oxacillin. For MSSA bacteremia, anti-staphylococcal beta-lactams (such as cefazolin, nafcillin, or oxacillin) are the drugs of choice. Clinical studies have consistently shown that beta-lactams provide superior bactericidal activity, leading to lower mortality rates, faster microbial clearance, and fewer relapses compared to vancomycin for MSSA infections. Therefore, the empiric broad-spectrum regimen should be de-escalated to targeted therapy with intravenous cefazolin.

Step-by-Step Solution

1
Interpret the blood culture susceptibility results.
The organism is Staphylococcus aureus that is susceptible to oxacillin, confirming a methicillin-susceptible S. aureus (MSSA) infection.
Oxacillin susceptibility is the laboratory surrogate marker used to differentiate MSSA from MRSA.
2
Identify the standard of care for MSSA bloodstream infections.
Anti-staphylococcal beta-lactams (e.g., cefazolin, nafcillin, oxacillin) are the first-line agents for MSSA.
Beta-lactams offer superior bactericidal activity, faster clearance of bacteremia, and lower mortality rates compared to vancomycin for MSSA infections.
3
Apply principles of antibiotic stewardship to modify the empiric regimen.
The empiric vancomycin and piperacillin-tazobactam should be discontinued and replaced with targeted cefazolin monotherapy.
De-escalating to the narrowest, most effective agent prevents antimicrobial resistance, limits collateral damage to normal flora, and reduces the risk of adverse drug events.

Key Concept

For methicillin-susceptible Staphylococcus aureus (MSSA) bacteremia, empiric broad-spectrum antibiotics should be de-escalated to an anti-staphylococcal beta-lactam (e.g., cefazolin, nafcillin) because these agents have superior clinical outcomes compared to vancomycin.
Estimated Time:1m 15s
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Infectious Diseases

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