Background: Vietnam National Children’s Hospital is a tertiary paediatric hospital. After two years of monitoring vancomycin levels using trough concentration (Ctrough), we developed a new therapeutic monitoring procedure using first-order pharmacokinetic-derived AUC (AUCPK), as recommended by the 2020 consensus guideline.1 Method: A retrospective study. Children older than 1-month-old admitted to the intensive care unit using vancomycin were included. Demographic data, vancomycin dosing regimen, serum concentrations, and dose adjustment were recorded. Patients’ vancomycin AUCs were calculated and compared between the first-order pharmacokinetic method (AUCPK) and the Bayesian-derived one (AUCBayes) using the Le et al model.2 The outcomes are target achievement rate; level of agreement of target attainment between AUCPK- and Ctrough-derived procedures; and level of agreement between different AUC calculating methods. Result: 59 patients were included. The median age was 10 months old. The median vancomycin duration was 9 days. The target achievement rate of AUCPK at the initial maintenance dose was 32.2%, increased gradually after dose adjustments and ultimately reached 100% after the third adjustment. 81.3% of therapeutic monitoring events fell within the same therapeutic categories between the Ctrough and AUCPK procedures. AUCPK and AUCBayes estimations using two concentrations (AUCBayes.2pts) showed a high extent of agreement with a mean difference of -5.27. Compared to AUCBayes.2pts, AUCBayes estimation using only peak or trough concentration showed certain differences, particularly in subtherapeutic and supratherapeutic ranges. Conclusion: Vancomycin AUC target achievement rate was low at the initial dose, but gradually improved. AUCPK and AUCBayes.2pts methods help avoid pitfalls in result interpretation experienced using Ctrough.