Background: Infectious complications are common after percutaneous nephrolithotomy (PCNL). Clinical guidelines recommend, previous to surgery, prolonged antibiotic regimens in patients with preoperative positive urine cultures to reduce infectious risk. However, such strategies may increase selective pressure and promote antimicrobial resistance. Evidence supporting the use of a single antibiotic dose tailored to culture sensitivity in these cases is limited but emerging. Methods: We conducted a retrospective observational study including 187 PCNL procedures performed between 2021 and 2023 under an individualized antibiotic prophylaxis protocol. Patients with negative or contaminated urine cultures received a single empirical dose, while those with recent positive cultures received a single dose based on antimicrobial susceptibility testing. Postoperative complications—including fever, sepsis, and a composite outcome—were analyzed through multivariable logistic regression, comparing high- and low-risk patients. Results: A total of 67.9% of procedures were performed in patients meeting at least one high-risk criterion, including a positive preoperative urine culture in 32.1%. The overall incidence of infectious complications was 11.9% (fever 8.7%, sepsis 3.2%), with no significant differences between risk groups. A low concordance was observed between preoperative and intraoperative urine cultures (Spearman = 0.3954). Conclusions: A single preoperative antibiotic dose adjusted to the antibiogram, even in patients with a positive urine culture, was not associated with increased infectious complications. This approach is an initial step that supports a rational and individualized prophylactic strategy aligned with the goals of antimicrobial stewardship programs (ASPs).