What does the term R4S indicate in quality control?

The term R4S indicates a situation in quality control where two controls in the same run are greater than 4 standard deviations apart. This is a critical scenario that signals a potential issue in the analytical process, suggesting that the system may not be operating within the expected range of variation. In essence, a difference of more than four standard deviations typically signifies that there is a significant discrepancy that cannot be attributed to normal variation, prompting further investigation.

This principle is employed to help maintain the reliability of laboratory results. Observing two consecutive controls that are this far apart raises red flags, leading to the need for corrective actions, such as recalibrating instruments, retraining personnel, or identifying potential issues with reagents.

While the other options present various scenarios of control limits and statistical quality control concepts, they do not correctly encapsulate the definition associated with R4S, which specifically refers to that substantial difference in values indicating a serious concern for quality control.