A control result flags on the morning run

It is 07:15 and the level 1 chemistry control has just posted at 104 mg/dL against a target mean of 100 mg/dL and an SD of 2 mg/dL. That places the point at plus 2 SD, inside the accepted range for many local rule sets but outside the band where results have been sitting all week. Whether the analyzer stops or simply reports the value depends on its configuration; the technologist still has to decide whether to hold and investigate under the current procedure before releasing a single patient result from this run.

A Levey-Jennings chart puts every control result you have run on a laboratory method on one time-ordered graph, plotted against the target mean and against lines drawn at plus and minus 1, 2, and 3 SD. One flagged point is a place to look. A pattern across several points is a place to think. The chart also shows an event: reagent lot R18 was installed the same morning the level 1 point moved.

What a control chart can and cannot tell you: whether the axes, the mean, and the SD lines are read correctly, whether an isolated extreme can be told apart from a shift, a trend, increased scatter, or a cyclical pattern, and whether the timing of a pattern connects to a documented event such as a reagent lot change, a calibration, maintenance, or an operator change, without turning the chart itself into proof of what caused it.

Reading a two-level chart across a reagent lot transition means naming the pattern, identifying the first date the evidence changed, and choosing the event record that is most relevant to inspect next. A flagged control point tells you to look at the chart before you decide anything about the run.