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Westgard rules calculator and Levey-Jennings chart

Paste your control results, set the target mean and SD, and see which Westgard rules fire, on a Levey-Jennings chart for each level. Free, no sign-up, and nothing you enter leaves your browser.

Free tool · Runs in your browser · Updated October 2026

1. Your control results

One value per line for a single level, or level,value, or date or run,level,value for up to three levels. A header row is fine; it is skipped.

Control results

2. Target mean and SD

Use your laboratory's established mean and SD (commonly from at least 20 results), or the control manufacturer's values until you have your own.

3. Rules

Sigma metric with CLIA allowable error

Pick an analyte to load its CLIA acceptance limit, then add your target concentration, CV% and bias% to get the sigma metric and a suggested rule set.

Analyte

Target concentration

CV%

Bias% (optional)

How to read the result

Rule	What it means	Usually points to
1-2s	One result beyond 2 SD	Warning: look at the other rules before acting
1-3s	One result beyond 3 SD	Random error, or a large systematic shift
2-2s	Two consecutive results beyond 2 SD on the same side	Systematic error
R-4s	In one run, one level above +2 SD and another below -2 SD	Random error
4-1s	Four consecutive results beyond 1 SD on the same side	Systematic error
10x	Ten consecutive results on the same side of the mean	Systematic error (a drift or shift)

What to do when a rejection rule fires

Do not simply repeat the control and accept the second result.
Use the rule that fired to decide between random and systematic error.
Check the usual causes: reagent or control lot changes, calibration, how the control material was stored and prepared, and recent maintenance.
Correct the cause, rerun the controls, and record what you found and did.
Review the patient results reported since the last accepted run, following your laboratory's procedure.

For the statistics behind each rule, read Westgard rules explained for POCT teams and Levey-Jennings charts and Westgard rules in POCT. To run these rules automatically on every QC result across all your analysers, with lockout when a rule fails, see Catenix quality control.

This tool helps with QC statistics only. It does not interpret patient results, and QC acceptance decisions stay with your laboratory. The engine is open source under the MIT licence (licence).

Questions, answered

Common questions about Westgard rules.

What are Westgard rules?

A set of statistical rules for deciding whether a run of quality control results is in control. Each rule looks for a pattern on the Levey-Jennings chart, such as one result beyond 3 SD (1-3s) or two in a row beyond 2 SD on the same side (2-2s), that points to random or systematic error.

Which Westgard rules should I use?

It depends on how good the method is. A high-sigma method can run on 1-3s alone; a lower-sigma method needs the full multirule and more control results per run. The sigma helper on this page suggests a starting point from your CV, bias and the CLIA allowable error.

Is my QC data sent anywhere?

No. The calculator runs entirely in your browser. Nothing you type or paste is uploaded or stored by Catenix.

Where do the CLIA limits come from?

From the acceptance limits in 42 CFR Part 493 Subpart I as amended by the CMS final rule CMS-3355-F (87 FR 41194, July 2022), effective 11 July 2024 and used by laboratories and proficiency testing providers from 1 January 2025. The rule text is authoritative.