Why are three levels used for quality control of pH and blood gases?

Using three levels for quality control of pH and blood gases is crucial as it allows for the earlier detection of systematic errors. Systematic errors can occur due to instrument drift, changes in calibration, or reagent degradation, which may not be immediately noticeable with just two controls. By incorporating three different levels, operators can monitor the system's performance more comprehensively across a wider range of expected values. This practice not only facilitates the identification of trends or shifts in measurement accuracy, but it also enhances the reliability of results, ensuring that they reflect true physiological conditions accurately and consistently.

The rationale behind maintaining high, normal, and low controls is that it provides a robust framework for assessing the accuracy and precision of the measurements throughout the full spectrum of expected physiological ranges. This is especially important for pH and blood gas analyses, where variations can indicate critical patient conditions. The other choices do not fully encapsulate the primary benefit of using three levels, thereby making them less suitable for this specific question about quality control in this context.