Vancomycin and tobramycin clearance in an infant during continuous hemofiltration

OBJECTIVE: To report a case of vancomycin and tobramycin clearance by continuous veno-venous hemofiltration in an infant. Hemofiltration clearance (ClHF) was calculated by two methods and compared for ease and reliability. METHODOLOGY: Case report of a hospitalized four-month-old infant. With method A, ClHF calculation for vancomycin and tobramycin was determined by accurate collection of ultrafiltrate in five 24-hour periods and a midpoint serum sample. With method B, ClHF calculation was determined by obtaining prefilter sample, postfilter sample, and blood flow through filter (Fick principle) over three study periods, correlating to three of five study periods in method A. RESULTS: The infant received continuous veno-venous hemofiltration. With method A, vancomycin ClHF ranged from 0.27 to 0.80 mL/min; tobramycin ClHF ranged from 0.32 to 0.91 mL/min. With method B, ClHF for vancomycin ranged from 0 to 2.08 mL/min. Tobramycin ClHF ranged from 0 to 1.6 mL/min when calculated with method B. CONCLUSIONS: Continuous veno-venous hemofiltration increased the clearance of vancomycin and tobramycin requiring dosage modifications. It appears that method A, which uses the ultrafiltration concentration compared with the serum concentration is more accurate than method B, as it averages fluctuations in ultrafiltrate flow rates. Method B compares a single pre- to postfilter drug concentration and relies on an accurate measurement of ultrafiltration flow rate. Determining ClHF based upon one point in time may overestimate ClHF when the ultrafiltration flow rate varies, as it does in the critically ill. Daily serum concentrations for vancomycin and tobramycin are recommended during continuous veno-venous hemofiltration.

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