Aim: Preanalytical errors, particularly during venous blood collection, are a significant source of laboratory diagnostic inaccuracies, accounting for most laboratory mistakes. Addressing these errors through systematic monitoring and targeted interventions is critical for improving laboratory quality and ensuring patient safety.

Material and Methods: This prospective study was conducted over five days in the central blood collection unit of a university hospital. A total of 300 venous blood collection procedures performed by 12 phlebotomists were directly observed. Data were collected using a Venous Blood Collection Observation Form based on the European Federation of Clinical Chemistry and Laboratory Medicine and Clinical and Laboratory Standards Institute guidelines. Compliance with guidelines and common error rates were analyzed, and the impact of training interventions was assessed through statistical methods, including the two-proportion z-test and chi-square test.

Results: The most common reasons for sample rejection were "Insufficient Sample," "Clotted Sample," and "Hemolyzed Sample," which accounted for 78.69% of all rejections. After the implementation of a training program in June 2024, a statistically significant reduction in sample rejection rates was observed in July 2024 (p < 0.05). The primary areas of non-compliance involved improper tube filling, incorrect mixing, and extended tourniquet application.

Conclusion: The findings demonstrate that preanalytical errors, particularly in venous blood collection, can be reduced through systematic training and adherence to standardized protocols. Continuous education and quality control measures are essential for minimizing error rates, improving sample integrity, and enhancing patient safety. Future efforts should focus on regular observational audits and ongoing staff education to maintain improvements.