The MKSAP Challenge
Two years after undergoing mitral valve replacement, a 48-year-old man has a cerebrovascular accident
Two years after undergoing mitral valve replacement, a 48-year-old man has a cerebrovascular accident. Except for fever, general physical examination is noncontributory.
The cardiac examination is unchanged from previous findings. A transesophageal echocardiogram shows an oscillating mass on the mitral valve but no evidence of perivalvular extension or abscess. Six sets of blood cultures grow Enterococcus faecalis, which is resistant to penicillin and ampicillin but sensitive to vancomycin. The laboratory also reports the absence of high-level resistance to gentamicin and streptomycin.
A decision is made to treat medically, and the patient is begun on vancomycin, 1 g intravenously every 12 hours, and gentamicin, 80 mg intravenously every eight hours (the patient weighs 76 kg [167 lb]). After three days of therapy, the laboratory reports that his vancomycin peak level is 32 µg/mL with a trough level of 9 µg/mL, and his gentamicin peak level is 3.2 µg/mL with a trough level of 0.8 µg/mL. The technician notes that the vancomycin peak and trough levels and the gentamicin trough level are in the desirable range but that the laboratory's therapeutic peak range for gentamicin is 4 to 8 µg/mL. Repeat blood cultures show no growth, and complete blood count and serum creatinine values are normal.
Which of the following is most appropriate at this time?
A. Increase the gentamicin dose; keep the vancomycin unchanged
B. Decrease the interval between the gentamicin doses; keep the vancomycin unchanged
C. Keep both the gentamicin and the vancomycin doses unchanged
D. Increase the vancomycin dose; keep the gentamicin unchanged
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