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Background and Importance Recent guidelines from the Infectious Diseases Society of America (IDSA) recommend early vancomycin monitoring, adjusting the dose to achieve an AUC24h of 400–600 mg·h/L. However, the time required for a patient to reach this target is not well established, despite its relevance to treatment efficacy and safety.Aim and Objectives To evaluate AUC24h levels during the first three days of vancomycin treatment in patients managed according to routine clinical practice with pharmacokinetic monitoring.Material and Methods Prospective, observational, single-centre study including 30 patients treated with vancomycin between January and February 2025. Data collected included weight, height, age, sex, serum creatinine (Scr) and administered doses. Individual pharmacokinetic parameters were estimated using Abbott’s PKS Bayesian software, and steady-state AUC24h was calculated to guide dosing recommendations. The pharmacokinetic profile was segmented by treatment days and digitally analysed using pixel quantification in a Python environment to determine actual AUC24h values during the first three days. The percentage of patients within the therapeutic range was calculated, with results expressed as mean and 95% confidence interval (CI 95%).Results Among the 30 patients (mean age 66 years (59–73), height 167 cm (164–170), weight 69 kg (64–74), 27% female (14–45), Scr 0.9 mg/dL (0.8–1.0)), only 23% (8–38) received a loading dose. On day 1, 20% (CI95: 6–34) reached the target AUC24h; 67% (50–84) were underdosed, 13% (1–25) overdosed. On day 2, 70% (54–86) were within range; 13% (1–25) underdosed, 17% (3–30) overdosed. On day 3, 64% (47–82) had optimal levels; 7% (0–17) underdosed, 28.6% (12–45) overdosed. Mean AUC24h values were 372 (302–441), 513 (464–561), and 543 (502–583) mg·h/L, respectively.Conclusion and Relevance Despite pharmacokinetic monitoring, 80% of patients had inadequate dosing on day 1, and one-third remained outside the therapeutic range after 72 hours. These deviations may compromise treatment efficacy and safety during the early phase, highlighting the need for early optimisation through loading doses, pharmacist validation, and timely monitoring.Conflict of Interest No conflict of interest