Background Obese patients with normal serum creatinine have increased renal clearance, and consequently the dose of some drugs, such as vancomycin, should be dosed based on actual body weight (ABW). According to the American Society of Health-System Pharmacists (ASHP) guidelines, the recommended dosing regimen is 15–20 mg/kg ABW/12 hours intravenously, with subsequent dosage adjustment based on serum vancomycin concentrations. Nevertheless, in our hospital, vancomycin is often used as a fixed dose regimen regardless of patient weight, based on the manufacturer´s labelling recommendation.
Purpose To determine the frequency of underdosing of vancomycin in obese patients and possible risk factors.
Material and methods A single centre, retrospective, observational study from January 2014 to September 2016 was carried out. Morbidly obese adult patients, defined as a body mass index (BMI) ≥ 30 according to the WHO classification, with at least one trough level obtained at steady state, were included. Patients were excluded if they had a creatinine clearance (CrCl) <35 mL/min, calculated by the Salazar–Corcoran formula. The therapeutic level for serious infections (endocarditis, osteomyelitis, meningitis, nosocomial pneumonia by Staphylococcus aureus, methicillin resistant Staphylococcus aureus bacteraemia) was 15–20 mg/mL, whereas for uncomplicated infections the optimal interval was 10–15 mg/mL, according to the ASHP guidelines. Binary logistic regression was done to identify variables associated with underdosing, using SPSS 15.0.
Results 46 patients were included, 63% women. Mean age±SD=70.78±12.5 years; mean weight±SD=87.8±13.9 kg; and mean height 1.59±0.09 m. 34.8% were underdosed. No association was found between gender and underdose (p=0.143), or BMI classification (p=0.679) or creatinine (p=0.079). On the other hand, statistical analysis suggested a relationship between underdose and age >65 years (OR 0.206, 95% CI 0.04–0.98; p< 0.05) and initial dosing regimen of 1000 mg/12 hours (OR 0.008, 95% CI 0.0–0.55; p< 0.05) compared with 1000 mg/24 hours.
Conclusion It is important to monitor levels of vancomycin in obese patients, especially in those aged <65 years and with an initial dosing regimen not adjusted to patient weight. Even so, there are currently insufficient data to make statements to guide vancomycin dosing in obese patients, so it is necessary for more studies to focus on this issue.
References and/or acknowledgements American Society of Health-System Pharmacists (ASHP) guidelines.
No conflict of interest
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