Methodological Study of Vancomycin Dosing in Elderly Patients Using Actual Serum Creatinine Versus Rounded Serum Creatinine

PURPOSE: The practice of intentional rounding up of serum creatinine (SCr) in elderly patients with low measured values can lead to an underestimation of creatinine clearance and subsequent inaccurate dosing of medications. Thus, the purpose of this study was to evaluate the accuracy of vancomycin dose calculations for patients aged ≥65 years using an SCr rounded up to 1 mg/dL versus actual SCr. METHODS: This study compared the difference between measured steady-state vancomycin trough concentrations with predicted trough concentrations that have been calculated using rounded SCr and actual SCr. All non-obese patients ≥65 years of age with a baseline SCr <1 mg/dL who received a vancomycin regimen based on a SCr rounded to 1 mg/dL, and had a steady-state trough drawn between June 2014 and December 2015, were evaluated. A total of 1709 patients were evaluated, of whom 56 met all the selection criteria. RESULTS: The mean difference between measured vancomycin trough concentrations and predicted trough concentrations using rounded SCr was 8.84 versus 4.54 μg/mL using actual SCr [mean difference 4.31, 95% confidence interval (CI) 3.2–5.41; p < 0.0001]. In female patients, the mean difference between measured concentrations and predicted concentrations using rounded SCr was 9.68 versus 3.53 μg/mL using actual SCr (mean difference 6.15, 95% CI 4.42–7.88; p < 0.0001), while in male patients the mean difference between measured concentrations and predicted concentrations using rounded SCr was 8.21 versus 5.29 μg/mL using actual SCr (mean difference 2.92, 95% CI 1.6–4.24; p < 0.0001). CONCLUSION: Using actual SCr to perform vancomycin dosing calculations more accurately predicted measured vancomycin troughs than using an SCr rounded to 1 mg/dL. In our sex-specific analysis, using actual SCr resulted in more accurate trough projections for both males and females than using a rounded SCr.