Vancomycin Underdosing Very Common in the Obese

More than 70% of obese patients do not receive adequate doses of vancomycin, possibly resulting in subtherapeutic concentrations, according to Dallas-based researchers. Criteria that are more stringent suggest that this may also be true of a much wider group of patients.

In the June issue of the American Journal of Medicine, Dr. Ronald G. Hall II of Texas Tech University and colleagues note that although body weight is the preferred method of calculating vancomycin dosage, many studies continue to use a flat dosing regimen of 1 g every 12 hours to evaluate new agents for the treatment of gram-positive infections, including methicillin-resistant Staphylococcus aureus.

In addition, the team points out, low concentrations of vancomycin appear to favor the emergence of vancomycin resistance.

To gain further information on actual clinical practice, the researchers examined data on a random sample of 421 patients who had been treated with vancomycin during a 3-year period. No institutional vancomycin-dosing policies were in effect at the two tertiary care centers from which patients were drawn.

Regardless of body mass index, most patients received a fixed dose of 2 g daily. Adequate dosing of at least 10 mg/kg per dose was achieved in all underweight patients, 99% of normal weight patients, 93.9% of overweight patients and 27.7% of obese patients.

However, using a target of 15 mg/kg per dose or more, as recommended by several US organizations, 97% of underweight patients received an adequate dose, as did only 46% of normal weight patients, 1% of those who were overweight and 0.6% of the obese.

There was little in the way of correction, as only 3.3% of patients receiving less than 10 mg/kg per dose had their regimen changed in the first 24 hours of therapy.

In light of these findings and given that about a third of Americans are obese, the researchers conclude that “greater efforts should be undertaken to ensure patients receive weight-based dosing because inadequate dosing can lead to subtherapeutic concentrations and potentially worse clinical outcomes.”

Am J Med 2008;121:515-518.

Reviewed by Ramaz Mitaishvili, MD

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