Westgard Rules in Clinical Laboratory Quality Control

A practical guide with clear Levey-Jennings charts so laboratory technicians can easily understand and apply each rule.

What Are Westgard Rules?

Westgard Rules are a set of statistical rules used in clinical laboratories to decide whether a quality control (QC) run is acceptable or must be rejected. They help detect both random error (imprecision) and systematic error (bias or shift).

Control results are plotted on a Levey-Jennings chart. The chart shows the mean and limits at ±1 SD, ±2 SD and ±3 SD. Westgard Rules look at how far and in what pattern the points move away from the mean.

How to Read the Charts Below

All charts use a control with Mean = 100 and 1 SD = 3.

Black dashed line = Mean
Yellow dashed lines = ±1 SD
Red dashed lines = ±2 SD and ±3 SD
Red points = Rule violation

1_2s

WARNING

One control result exceeds ±2 SD. This is only a warning. Testing can continue, but watch the next results carefully.

1_3s

REJECTION

One control result exceeds ±3 SD. This is a rejection. Stop testing immediately and investigate.

2_2s

REJECTION

Two consecutive results exceed the same ±2 SD limit (same side of the mean). Indicates systematic error.

R_4s

REJECTION

The difference between two controls in the same run is greater than 4 SD (one high, one low). Indicates random error / imprecision.

4_1s

REJECTION

Four consecutive results exceed the same ±1 SD limit on the same side of the mean. Shows a developing systematic shift.

10_x

REJECTION

Ten consecutive results fall on the same side of the mean. Indicates persistent systematic bias.

How Technicians Should Apply the Rules

Plot every control result on the Levey-Jennings chart.
First check the 1_2s rule (warning).
If 1_2s is violated → immediately check the rejection rules (1_3s, 2_2s, R_4s, 4_1s, 10_x).
If any rejection rule is violated → stop patient testing, investigate, correct the problem, and re-run controls.
Document the violation, the cause found, and the corrective action taken.

Quick Reference Table for Technicians

Rule	What You See on Chart	Action
1_2s	1 point beyond ±2 SD	Warning – continue testing, watch next results
1_3s	1 point beyond ±3 SD	Reject run – investigate
2_2s	2 consecutive points beyond same ±2 SD	Reject run – systematic error
R_4s	Range between 2 controls > 4 SD	Reject run – random error
4_1s	4 consecutive points beyond same ±1 SD	Reject run – systematic shift
10_x	10 points on same side of the mean	Reject run – systematic bias

Educational Disclaimer: This page is for training and reference purposes only. Always follow your laboratory’s official QC policy and regulatory requirements.