A 35-year-old IVDU presents with high fever, septic emboli to lungs, and a new pansystolic murmur at the left sternal edge increasing with inspiration. Blood cultures are positive for Staphylococcus aureus. Modified Duke Criteria are met. Echocardiography shows tricuspid valve vegetation 1.5 cm with severe tricuspid regurgitation. The MOST appropriate antibiotic regimen is:
- A IV vancomycin 25-30 mg/kg/day in 2 divided doses (empirical until sensitivities available) ✓
- B IV nafcillin/oxacillin 2g 4-hourly for 6 weeks (MSSA)
- C IV ceftriaxone 2g daily for 4 weeks (right-sided IVDU endocarditis)
- D Oral linezolid 600 mg twice daily (for right-sided endocarditis)
Explanation
In IVDU-associated endocarditis, Staphylococcus aureus (including MRSA) is the most common organism. Pending culture and sensitivity results, empirical treatment for suspected MRSA is IV vancomycin. Once MRSA is confirmed, vancomycin is continued. If MSSA is confirmed, anti-staphylococcal penicillin (nafcillin, oxacillin, or flucloxacillin) is preferred over vancomycin for MSSA. Duration for right-sided uncomplicated MSSA/MRSA endocarditis is 4 weeks (some cases 2-week oral options in MSSA right-sided disease only per POET trial data). IV ceftriaxone is used for streptococcal endocarditis. Oral linezolid has activity but is not first-line for S. aureus endocarditis.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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