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Difficulty: HardInfectious Diseases

A 48-year-old man is hospitalized with a 1-week history of fever and severe lower back pain. He has a history of injection drug use. Temperature is 38.6°C (101.5°F), blood pressure is 120/75 mm Hg, and pulse is 98/min. Physical examination shows point tenderness over the lower lumbar spine. Neurologic examination is unremarkable. Magnetic resonance imaging of the spine reveals L3-L4 vertebral osteomyelitis and a small epidural abscess without cord compression. Empiric therapy with intravenous vancomycin and piperacillin-tazobactam is initiated. Two days later, blood cultures drawn at admission grow methicillin-susceptible Staphylococcus aureus (MSSA). The isolate is susceptible to oxacillin, cefazolin, and vancomycin. The patient's clinical status is stable. Which of the following is the most appropriate next step in pharmacotherapy?

  1. A
    Discontinue piperacillin-tazobactam and continue intravenous vancomycin
  2. B
    Continue intravenous vancomycin and piperacillin-tazobactam
  3. Discontinue vancomycin and piperacillin-tazobactam, and initiate intravenous cefazolinAnswer
  4. D
    Discontinue vancomycin and piperacillin-tazobactam, and initiate oral cephalexin
  5. E
    Discontinue vancomycin and continue intravenous piperacillin-tazobactam

Answer

The most appropriate step is to discontinue vancomycin and piperacillin-tazobactam, and initiate intravenous cefazolin.
The correct action is to transition the patient to a targeted, narrow-spectrum beta-lactam such as intravenous cefazolin. In patients with serious infections caused by methicillin-susceptible Staphylococcus aureus (MSSA), including bacteremia, endocarditis, and osteomyelitis, anti-staphylococcal beta-lactams (e.g., cefazolin, oxacillin, nafcillin) are the treatment of choice. Clinical data consistently demonstrate that these agents are superior to vancomycin for MSSA, resulting in faster clearance of bacteremia, lower rates of clinical failure, and reduced mortality. Therefore, empiric vancomycin must be discontinued. Furthermore, continuing piperacillin-tazobactam is inappropriate; while it covers MSSA, it is an overly broad-spectrum agent that disrupts normal flora and selects for resistant organisms. Antibiotic stewardship dictates narrowing the spectrum of coverage as soon as culture and susceptibility data are available.

Step-by-Step Solution

1
Identify the patient's diagnosis and the responsible pathogen.
The patient has vertebral osteomyelitis, an epidural abscess, and bacteremia caused by methicillin-susceptible Staphylococcus aureus (MSSA).
Establishing the definitive microbiological etiology allows for the transition from empiric to targeted antimicrobial therapy.
2
Evaluate the efficacy of the current empiric antibiotic regimen against the identified pathogen.
While both vancomycin and piperacillin-tazobactam provide in vitro coverage for MSSA, they are suboptimal choices for definitive therapy.
Vancomycin is clinically inferior to beta-lactams for MSSA, and piperacillin-tazobactam is unnecessarily broad, which promotes antimicrobial resistance.
3
Select the optimal targeted therapy for a serious MSSA infection.
Intravenous cefazolin or an anti-staphylococcal penicillin (such as oxacillin or nafcillin) should be initiated.
Narrow-spectrum beta-lactams demonstrate superior clinical outcomes, including faster bacterial clearance and lower mortality, for deep MSSA infections compared to vancomycin.

Key Concept

For serious methicillin-susceptible Staphylococcus aureus (MSSA) infections (e.g., osteomyelitis, endocarditis, bacteremia), targeted therapy with an anti-staphylococcal beta-lactam (such as cefazolin, oxacillin, or nafcillin) is clinically superior to vancomycin and should be initiated once susceptibilities are known.
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Infectious Diseases

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