In today's health care system the prevalence of medical errors is high as stated by the report of the Institute of Medicine. A varying error rate of <10% in clinical medical laboratories has been reported in the literature. Most of these errors occur in the pre-analytical phase. Only a small number of errors will be seen in the analytical phase. This overview will deal with the analytical interferences and will offer ways to improve the analytical quality. Some special areas of the analytical process like calibration, quality control, reference interval, drug interference, statistical analysis and volume displacement will be covered. With some examples from the literature and own investigations the impact of errors in these steps of the analytical process will be better understood and the examples will help reducing the number of analytical errors and interferences. This finally provides better patient safety.