Levey-Jennings charting is a statistical process control method used in clinical laboratories to visualise the performance of a test or instrument over time by plotting control values against established limits. For respiratory labs running spirometry, it is one of the most practical tools available for detecting when results are drifting, shifting, or simply no longer trustworthy. Done consistently, it transforms spirometry quality control from a reactive, incident-driven process into a proactive, data-driven one.

TL;DR

• A Levey-Jennings chart plots repeated control measurements over time against mean and standard deviation limits, making instrument performance visible at a glance [westgard.com].

• It detects two types of error: random error (precision problems) and systematic error (accuracy drift or bias) [ncbi.nlm.nih.gov].

• Spirometry quality assurance depends on consistent, longitudinal monitoring, and Levey-Jennings charting provides the structure to do that.

• Westgard Rules are applied alongside the chart to define exactly when to act on a warning or reject a run [diamonddiagnostics.com].

• Purpose-built respiratory lab software like Rezibase has built-in quality control tooling aligned to Westgard methods, removing the need for manual spreadsheet tracking.

About the Author: This article was written by the Rezibase team, a platform founded by respiratory scientists and trusted by over 35 clinical physiology sites across Australia and the UK, including NHS and NSW Health sites. Rezibase's quality control module is built specifically around the Westgard and Levey-Jennings methods used in respiratory and sleep labs.

What Is a Levey-Jennings Chart and How Does It Work?

A Levey-Jennings chart is a type of statistical process control (SPC) chart adapted for laboratory quality control [leansixsigmadefinition.com]. It displays control measurements on the Y-axis and time (typically run sequence or date) on the X-axis. Horizontal lines are drawn at the mean, and at one, two, and three standard deviations (SD) above and below the mean [westgard.com].

The visual logic is straightforward:

• Most results from a stable process should fall within ±2 SD.

• A result beyond ±3 SD suggests something has gone wrong.

• A series of results trending in one direction, even within limits, can signal a developing problem [scribd.com].

What makes the Levey-Jennings chart valuable is not any single data point. It is the pattern. A single outlier might be noise. Five consecutive results on one side of the mean is a shift. A steady climb toward the upper limit is a trend. These patterns are invisible without a time-series view [medlabacademy.com].

What Types of Error Does Levey-Jennings Charting Detect?

There are two fundamental error categories in laboratory quality control, and a Levey-Jennings chart is sensitive to both [ncbi.nlm.nih.gov]:

Systematic error is particularly important in spirometry quality assurance because it can silently degrade result accuracy across an entire patient cohort before anyone notices during routine review.

Why Is Levey-Jennings Charting Relevant to Spirometry Specifically?

Spirometry is not a point-in-time test. Patients return for repeat testing. Clinicians rely on longitudinal comparisons to assess disease progression or treatment response. If an instrument has drifted by even a few percentage points between visits, a clinician may interpret a real decline as a measurement artefact, or miss a genuine change altogether.

This is why spirometry quality control requires more than just calibration checks at the start of the day. It requires a documented, visual record of instrument behaviour over time. The Levey-Jennings chart is the standard mechanism for providing exactly that [pmc.ncbi.nlm.nih.gov].

Research published in a peer-reviewed study found that Levey-Jennings analysis can uncover unsuspected sources of variability that would not be detected through routine visual inspection alone [pmc.ncbi.nlm.nih.gov]. The original context was immunohistochemistry, but the principle is directly applicable to any laboratory procedure that uses repeated control measurements, including pulmonary function testing.

How Do You Build and Interpret a Levey-Jennings Chart Template?

A functional Levey-Jennings chart template requires the following inputs [westgard.com]:

Step 1: Establish baseline statistics

• Run the same control material at least 20 times under normal operating conditions.

• Calculate the mean and standard deviation from those runs.

Step 2: Draw the chart

• Y-axis: control measurement values.

• X-axis: run number or date.

• Plot horizontal lines at: mean, mean ±1 SD, mean ±2 SD, mean ±3 SD.

Step 3: Plot ongoing results

• Each time a QC run is performed, plot the result on the chart.

Step 4: Apply Westgard Rules

• Use the chart in conjunction with Westgard Rules to determine whether a run should be rejected or flagged for review [diamonddiagnostics.com].

Step 5: Investigate and document

• When a rule violation is triggered, document the investigation and any corrective action taken.

Many labs start with a manual spreadsheet-based Levey-Jennings chart template, which works in the early stages. However, as the volume of QC data grows, manual charting becomes time-consuming and error-prone. This is where purpose-built software adds significant practical value.

What Are Westgard Rules and How Do They Relate to Levey-Jennings Charts?

Westgard Rules are a standardised set of statistical criteria used alongside Levey-Jennings charts to decide when to accept, warn, or reject a QC run [diamonddiagnostics.com]. They are not separate from the chart; they are the decision framework applied to what the chart shows.

Common Westgard Rules include:

• 1₂s: One control exceeds ±2 SD. Warning only. Do not reject on this rule alone.

• 1₃s: One control exceeds ±3 SD. Reject the run. Indicates random error.

• 2₂s: Two consecutive controls exceed ±2 SD on the same side. Reject. Indicates systematic error.

• R₄s: One control exceeds +2 SD and another exceeds -2 SD in the same run. Reject. Indicates random error.

• 4₁s: Four consecutive controls exceed ±1 SD on the same side. Reject. Indicates systematic error.

• 10x: Ten consecutive controls fall on the same side of the mean. Reject. Indicates systematic error [diamonddiagnostics.com].

Understanding which rule was violated helps identify what type of error occurred and guides the investigation that follows.

Frequently Asked Questions

What is a Levey-Jennings chart used for in a respiratory lab?
It is used to monitor the stability and precision of spirometry instruments over time by plotting QC results against established statistical limits [medlabacademy.com].

How often should QC be run in a spirometry lab?
Most accreditation standards require at minimum daily QC runs when the instrument is in use, though the exact frequency may depend on your local accreditation body's requirements.

Can a Levey-Jennings chart detect calibration problems?
Yes. A sudden shift in control results following a calibration adjustment or equipment service event is one of the clearest signals visible on a Levey-Jennings chart [scribd.com].

What is the difference between a warning and a rejection in Westgard Rules?
A warning (such as 1₂s) signals that something may be developing and warrants attention. A rejection rule (such as 1₃s) indicates the run results should not be reported until the issue is resolved [diamonddiagnostics.com].

Does Rezibase support Levey-Jennings quality control?
Yes. Rezibase's accreditation module includes quality control functionality built around Westgard methods, removing the need for manual charting and making longitudinal QC data accessible in one place.

Is spirometry quality assurance required for accreditation?
Yes. Standards such as ISO 15189 and TSANZ/NATA requirements include QC and documentation obligations that Levey-Jennings charting directly supports.

Can small labs implement Levey-Jennings charting?
Absolutely. The method scales to any lab size, and the core principles apply regardless of test volume [diamonddiagnostics.com].

About Rezibase

Rezibase is Australia's most advanced cloud-based respiratory and sleep reporting platform, built by respiratory scientists for respiratory scientists. Trusted by over 35 sites including NHS and NSW Health, Rezibase covers the full clinical workflow from patient administration and spirometry reporting through to accreditation management and quality control. Its built-in QC module aligns to Westgard methods and Levey-Jennings standards, helping labs meet ISO 15189 and TSANZ/NATA requirements without spreadsheets or manual workarounds. Rezibase is vendor-neutral, requires no local software installation, and is available on a transparent monthly subscription with no lock-in contracts.

Ready to move your spirometry quality control off spreadsheets and into a purpose-built system? Visit rezibase.com to learn more or start your 30-day free trial.