Levey-Jennings (LJ) charts are statistical quality control tools used in clinical laboratories to monitor whether a measurement process remains stable over time. By plotting control values against calculated mean and standard deviation limits, they make calibration drift and systematic errors visible before those errors reach patient results. For clinical physiology labs running spirometry, lung diffusion, or sleep studies, LJ charts are one of the most practical and accessible tools available for maintaining measurement integrity day to day.

TL;DR

• A Levey-Jennings chart plots QC values over time against mean ± 1SD, 2SD, and 3SD limits to detect when a process is drifting out of control.

• Westgard rules are applied to LJ charts to distinguish random error from systematic error using defined, objective criteria.

• Systematic errors are particularly dangerous in clinical physiology because they produce consistently wrong results across multiple patients before anyone notices.

• Westgard-based QC is a requirement under ISO 15189 and TSANZ/NATA accreditation standards for respiratory and sleep labs.

• Digital platforms like Rezibase can automate LJ chart generation and Westgard rule checking as part of a fully integrated accreditation workflow.

What Exactly Is a Levey-Jennings Chart?

A Levey-Jennings chart is a type of control chart specifically adapted for clinical laboratory use. It was developed by Stanley Levey and Elmer Jennings in the 1950s, drawing on industrial statistical process control principles and applying them to medical testing.

The chart works as follows:

• A stable control material with a known target value is measured repeatedly over time.

• Each result is plotted as a single point on the y-axis, with time (days, runs, or batches) on the x-axis.

• Horizontal lines mark the mean, ±1SD, ±2SD, and ±3SD limits calculated from a baseline period.

According to Westgard QC, the purpose is to construct the chart, plot control values, and then interpret those results systematically, identifying when a process has shifted or drifted outside acceptable limits.

What makes LJ charts different from generic run charts is that the control limits are statistically derived from the process itself, not set arbitrarily. This gives them genuine diagnostic power.

Why Do Systematic Errors Matter More Than Random Errors in Clinical Physiology?

Random errors are unpredictable fluctuations that scatter results in both directions. They are expected, manageable, and typically average out across a dataset.

Systematic errors are a different problem. They shift all results consistently in one direction, meaning every patient tested during a period of systematic error receives a result that is wrong in the same way. As noted by Diamond Diagnostics, LJ charts combined with Westgard rules help identify both systematic and random errors in laboratory testing, with systematic errors being particularly important to detect early.

In a respiratory lab, a systematic error in a spirometer calibration could mean:

These are not hypothetical risks. They are the exact scenarios that structured QC programs exist to prevent.

How Do You Build and Read a Levey-Jennings Chart? A Step-by-Step Tutorial

This levey-jennings chart tutorial covers the core construction process applicable to any clinical physiology QC program.

Step 1: Establish your baseline
Run your control material a minimum of 20 times under normal operating conditions. Calculate the mean and standard deviation from these values.

Step 2: Draw the control limits
Plot six horizontal reference lines: mean, +1SD, -1SD, +2SD, -2SD, +3SD, -3SD.

Step 3: Plot ongoing control values
Each time you run QC, plot the result. Use a consistent control material from the same lot.

Step 4: Apply Westgard rules to interpret the chart
Do not rely on visual impression alone. Apply the rules systematically (covered in the next section).

Step 5: Document and act
Any rule violation requires documentation, investigation, and corrective action before patient results are reported.

MedLab Academy notes that LJ charts also serve as a valuable teaching tool for new staff, helping illustrate the principles of precision and accuracy in a concrete, visual format. This dual function, both monitoring and training, makes them particularly valuable in busy clinical physiology departments.

What Are Westgard Rules and How Do They Apply to LJ Charts?

Westgard rules explained simply: they are a set of decision criteria applied to control chart data to determine whether a QC run should be accepted or rejected.

The most commonly applied rules are:

According to Quality Gurus, the LJ chart is specifically adapted for clinical laboratories and is most effective when paired with these structured decision rules rather than used as a standalone visual tool.

The power of this combination is that it removes subjectivity. A scientist does not need to guess whether a trend is concerning. The rules provide an objective answer.

What Do Levey-Jennings Chart Examples Look Like in Practice?

Levey-jennings chart examples help make the theory concrete. Three common patterns to recognise:

Pattern 1: Shift
Multiple consecutive points suddenly move to one side of the mean. This typically indicates a reagent lot change, recalibration event, or equipment service that altered the baseline. The 10x or 4₁s rules will flag this.

Pattern 2: Trend
Points progressively drift in one direction over several days. This is a hallmark of calibration drift, reagent degradation, or a slowly failing component. Catching this early is exactly what LJ charts are designed for.

Pattern 3: Increased scatter
Points remain within limits but show much wider variation than usual. This suggests a precision problem, possibly from an inconsistent technique, unstable reagent, or environmental factor.

Each pattern points toward a different root cause, which is why pattern recognition on LJ charts is a core competency for any clinical scientist.

How Does This Apply to Accreditation Requirements for Respiratory and Sleep Labs?

ISO 15189, the international standard for medical laboratory quality, requires documented QC programs that include ongoing monitoring of measurement stability. TSANZ and NATA standards for respiratory and sleep departments in Australia align with this requirement.

Diamond Diagnostics recommends that labs run daily internal QC, enrol in at least one external QC program, and implement peer comparison. LJ charts and Westgard rules are the practical mechanism through which these requirements are met and documented.

This is where Rezibase adds direct value. The platform includes a dedicated accreditation module built around Westgard methods, covering QC documentation alongside non-conformance management, audits, and action plans. Rather than managing LJ charts in spreadsheets, labs using Rezibase have this workflow embedded in the same system used for patient reporting.

Frequently Asked Questions

What is a Levey-Jennings chart used for?
It is used to monitor QC control values over time, making it possible to detect when a measurement process is drifting or has shifted out of its expected range.

How many data points do you need to set up an LJ chart?
A minimum of 20 control measurements is generally recommended to calculate a reliable mean and standard deviation for the control limits.

What is the difference between a warning rule and a rejection rule in Westgard?
A warning rule (such as 1₂s) signals that something may be developing and prompts closer attention. A rejection rule (such as 1₃s) means the run must be rejected and patient results must not be reported until the issue is resolved.

Can LJ charts detect calibration drift?
Yes. A gradual trend on an LJ chart, where values progressively move in one direction over days or weeks, is one of the clearest indicators of calibration drift.

Are LJ charts required for accreditation?
Under ISO 15189 and TSANZ/NATA standards, documented QC monitoring using statistically based methods is required. LJ charts paired with Westgard rules satisfy this requirement.

How often should QC be run in a respiratory lab?
Daily QC is the recommended best practice, with additional QC runs following any equipment service, reagent lot change, or recalibration event.

Can LJ chart management be automated?
Yes. Platforms such as Rezibase include built-in QC modules that automate chart generation and Westgard rule checking, reducing manual workload and documentation burden.

About Rezibase

Rezibase is a cloud-based respiratory and sleep reporting platform designed by respiratory scientists for respiratory scientists. Trusted by over 35 sites including NHS UK and NSW Health, it offers a fully integrated accreditation module built around Westgard QC methods, alongside streamlined reporting, normal values libraries, and vendor-neutral device integration. Learn more at rezibase.com.

Ready to see how Rezibase handles QC documentation, LJ charts, and Westgard rule tracking within a single accreditation workflow? Visit rezibase.com to book a demo or start a 30-day free trial.

References

• Westgard QC. QC: The Levey-Jennings Control Chart. https://www.westgard.com/lessons/basic-qc-practices-l/lesson12.html

• Scribd. Levey-Jennings Chart in QC Biochemistry. https://www.scribd.com/document/881964817/Quality-Control-Lj-Chart

• Diamond Diagnostics. Quality Control for Small Labs: Levey-Jennings Charts & Westgard Rules. https://www.diamonddiagnostics.com/blog/quality-control-for-small-labs-levey-jennings-charts-westgard-rules

• MedLab Academy. Understanding Levy-Jennings. https://www.medlabacademy.com/post/understanding-levy-jennings

• Quality Gurus. Levey-Jennings Chart. https://www.qualitygurus.com/levey-jennings-chart/