If one of two controls within a run is above +2s and the other is below -2s, what does this indicate?

When one control result is above +2 standard deviations (s) and the other is below -2s, it indicates that there are significant variations in the measurements from the expected range, suggesting a lack of precision in the method. Poor precision is characterized by random error, which can occur due to various factors such as instrument fluctuations, operator variability, or environmental conditions affecting the analytical process.

In this scenario, having one control significantly high and the other significantly low implies that the method's performance is not consistent. The random error leads to variability that results in control values that do not cluster closely around the mean. This situation suggests that the method may not reliably produce reproducible results under the testing conditions.

On the other hand, systematic error refers to a consistent deviation from the true value, while proportional error implies a consistent percentage deviation across the range. Contamination of quality control material could lead to an overall shift in results, but the specific pattern of one value being high and the other low aligns more closely with random error. Therefore, the presence of random error and the poor precision it indicates is the correct interpretation of this specific control situation.