The Westgard R 4S rule is a quality control rule in laboratory medicine. It is used to detect sudden, large errors in analytical runs. The rule is triggered when the difference between two control measurements within a run exceeds 4 standard deviations (SDs). This is a significant deviation and suggests a potential problem with the run. Therefore, it is not merely a warning that can be ignored; instead, it indicates that the run should be stopped and the problem identified and corrected before proceeding