Westgard Rules Explained
Westgard multi-rules are the decision criteria laboratories use on Levey-Jennings charts to decide whether a quality control run is in control or must be rejected. They combine a sensitive warning rule with several rejection rules that help distinguish random error from systematic error.
How to read the chart
Control results are plotted against the established mean, with lines at ±1, ±2 and ±3 standard deviations (SD). Each new result is checked against the rules in order, starting with the 1:2s warning.
The six rules
1:2s Warning · Either error
One control observation exceeds the mean ±2 SD. On its own it is a warning only: inspect the other rules and the run before deciding to accept or reject.
1:3s Rejection · Random (or large systematic) error
One control observation exceeds the mean ±3 SD. Reject the run, then look for a random error such as a bubble, short sample or reagent problem.
2:2s Rejection · Systematic error
Two consecutive control observations exceed the same limit (+2 SD or −2 SD), within one level or across levels. Suggests calibration or reagent drift.
R:4s Rejection · Random error
Within one run, one control exceeds +2 SD and another exceeds −2 SD, a range of more than 4 SD. Indicates increased imprecision.
4:1s Rejection · Systematic error
Four consecutive control observations exceed the same limit, +1 SD or −1 SD. An early sign of a developing shift or trend.
10:x Rejection · Systematic error
Ten consecutive control observations fall on the same side of the mean. Indicates a persistent shift, often after a new lot, calibration or maintenance.
Random vs. systematic error
Random error (1:3s, R:4s) affects precision and shows up as scatter. Systematic error (2:2s, 4:1s, 10:x) affects accuracy and shows up as a shift or trend. Knowing which rule fired points troubleshooting toward the right cause: pipetting, bubbles and mixing for random error; calibrators, reagent lots and instrument maintenance for systematic error.
Practice with the Westgard simulator
Apply these rules to Levey-Jennings charts and bench-floor troubleshooting scenarios. Free account required.
Open the Westgard simulatorEducational reference only. Always follow your laboratory's approved QC procedures and your instrument and reagent manufacturers' instructions.