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Critical Care Medicine

Answer 1
  1. Gentamicin 100 mg once daily. The half-life of aminoglycosides is considerably longer in the elderly because of decreased glomerular filtration with age. Because the diminution of glomerular function in the elderly is not necessarily reflected in a higher serum creatinine level, the calculated creatinine clearance is used to estimate the clearance of aminoglycosides. The clearance of aminoglycosides is linearly related to the endogenous creatinine clearance; therefore, the ratio of calculated to normal creatinine clearance closely approximates the ratio of aminoglycoside clearance. This patient’s calculated creatinine clearance is approximately 35 mL/min. Using a daily dose of 5 mg/kg of gentamicin (or tobramycin or netilmicin) this patient should have received a daily dose of (35/100) x 5 mg/kg x 55 kg = 96 mg (rounded to 100 mg). No clinical data exist to suggest that the incidence of nephrotoxicity is less with either tobramycin or netilmicin. Data from several clinical studies suggest that a once-daily dosing regimen is at least as efficacious as the standard dosing regimen and may be less toxic.1-3

    REFERENCES
    1. Hatala R, Dinh T, Cook DJ: Once-daily aminoglycoside dosing in immunocompetent adults: a meta-analysis. Ann Intern Med 1996;124:717-725.

    2. Prins JM, Buller HR, Kuijper EJ, et al: Once versus thrice daily gentamicin in patients with serious infections. Lancet 1993;341:335-339.

    3.  Moore RD, Smith CR, Lipsky JJ, et al: Risk factors for nephrotoxicity in patients treated with aminoglycosides. Ann Intern Med 1984;100:352-357.

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