Laboratory Operations

A Levey-Jennings chart plots control values over time (run or date on the x-axis, control concentration on the y-axis). It displays the established mean as a center line and horizontal lines at ±1 SD, ±2 SD, and ±3 SD to let the analyst visually detect shifts, trends, and out-of-control results.

CV (%) = (SD ÷ mean) × 100. It expresses imprecision as a percentage of the mean, allowing comparison of precision between methods, analytes, or instruments that have different concentration ranges. A lower CV indicates better precision.

Accuracy is the closeness of a measured value to the true (or target) value. Precision is the closeness of repeated measurements to one another (reproducibility), regardless of the true value. A method can be precise but inaccurate (tight cluster off-target) or accurate but imprecise.

The 1-3s rule is violated when a single control result falls outside ±3 SD from the mean. It is a rejection rule that detects random error. When violated, the run is rejected and patient results are not reported until the problem is corrected.

The 1-2s rule (one control outside ±2 SD) is used as a warning rule, not an outright rejection rule. It signals the analyst to inspect the data using other Westgard rules before deciding whether to accept or reject the run, reducing false rejections.

The 2-2s rule is violated when two consecutive control results exceed the same ±2 SD limit (both >+2 SD or both <-2 SD). It is a rejection rule that detects systematic error. It can be evaluated within a run (across control levels) or across consecutive runs.