Levey-Jennings Charts: Visualizing Daily QC Data

Category: Basic QC Concepts | Estimated reading time: 8 minutes

Why Numbers Alone Are Not Enough

From the preliminary testing process, we already have the Mean (TV), SD, UCL, and LCL — solid numbers established based on our laboratory's actual conditions. But numbers alone are not enough.

Imagine you have 30 daily QC results in the form of a list of numbers. Can you quickly see if there is a trend developing? Is there a shift occurring slowly? Is there a pattern that needs to be addressed before any official rule is violated?

The answer: it is very difficult if the data is just numbers. This is why Levey-Jennings charts exist — they transform rows of numbers into a visual representation that can be read, interpreted, and communicated in seconds.

A Brief History You Need to Know

This chart was first introduced by Stanley Levey and Elmer Jennings in 1950, in their paper "The Use of Control Charts in the Clinical Laboratory" published in the American Journal of Clinical Pathology.

The idea was not actually original to the clinical laboratory — Levey and Jennings adapted the Shewhart control chart concept developed by Walter A. Shewhart at Bell Laboratories in the 1920s for industrial manufacturing quality control.

What is brilliant about this adaptation: they realized that the same principle — that a stable process will produce output normally distributed around the mean, and deviations from that pattern are signals that something has changed — also applies to measurement processes in clinical laboratories.

More than 70 years later, the Levey-Jennings chart remains the most fundamental and universal QC visualization tool in medical laboratories worldwide. That alone proves that the concept behind it is truly solid.

Anatomy of a Levey-Jennings Chart

LJ charts have a consistent structure that must be understood before they can be read correctly:

• X-axis (horizontal) — time. This can be in days, runs, or shifts, depending on the frequency of the QC performed. Each point represents one measurement of control material at a specific time.
• Y-axis (vertical) — the measurement value of the control material, in the same units as the parameter being measured.

Horizontal lines that stretch across the chart:

Line Position Meaning Mean TV Target value — this is where the ideal QC points cluster ±1SD TV ± 1SD Approximately 68% of QC points are expected to be in this zone ±2SD TV ± 2SD Approximately 95% of QC points are expected to be in this zone — warning limits ±3SD TV ± 3SD (UCL/LCL) Approximately 99.7% of QC points are expected to be in this zone — hard reject limits Data points plotted each day connect to form a visual picture of how the process is behaving over time.

Three Patterns to Recognize

There are three basic patterns that every medical laboratory technologist (MLT) must recognize, and all three have very different meanings.

Normal

Points are scattered randomly around the mean — most within ±1SD, almost all within ±2SD, and none outside ±3SD. There is no consistent pattern; points go up and down without a predictable direction.

This is the expected appearance of a stable process. The variation seen is typical random error — small fluctuations that are a normal characteristic of any measurement system.

Trend / Shift

Points begin to move consistently in one direction — crossing +1SD, +2SD, and finally moving outside +3SD over a few days.

This is a classic display of developing systematic error. Something has changed systematically in the process, and that change is accumulating over time. The most common causes: calibration starting to drift, reagents nearing expiration, or improper storage conditions.

Important to note: these trend signals are usually visible long before any official Westgard rule is violated. An ATLM trained to read LJ charts will recognize these patterns early and take preventive action before the QC is actually rejected.

Outside ±3SD

A point that falls outside the UCL or LCL is a hard violation — the Westgard 1₃s rule is broken and the QC must be rejected. Patient results should not be released until the problem is identified and corrected.

Reading a Chart vs. Simply Looking at a Chart

This is a very important distinction — and it is what separates a competent MLT from one who simply follows procedures.

Simply looking at a chart means: checking whether any points are out of bounds. If none → accept, continue. If there are → reject, repeat.

Reading a chart means checking not only for the presence of violations but also whether there is a developing pattern — even before an official violation occurs. This includes:

• Do the points tend to be on one side of the mean consistently?
• Is there a gradual movement in one direction even if still within limits?
• Is the variation between points suddenly larger than usual?
• Is there a sudden change in the average level from one period to the next?

All these patterns are meaningful signals — even if no Westgard rule has been violated yet. The ability to read these signals is the core of proactive, rather than merely reactive, QC.

The LJ chart provides the picture. The Westgard Multirules provide the objective decision criteria. Both work together.

Practical: How to Build an LJ Chart

From the TV and SD established through preliminary testing, calculate the following six boundary lines:

Line Value +3SD TV + (3 × SD) +2SD TV + (2 × SD) +1SD TV + (1 × SD) Mean TV −1SD TV − (1 × SD) −2SD TV − (2 × SD) −3SD TV − (3 × SD) Create a chart with the X-axis = time and the Y-axis = measurement value, draw the seven horizontal lines above, then plot one point each day based on the QC value obtained and connect it to the previous point.

In modern practice, almost all automated laboratory instruments have built-in QC modules that generate LJ charts automatically. But understanding how to build them manually remains an essential foundation — without understanding the structure, the automatically generated charts will just be pictures without meaning.

Closing

The Levey-Jennings chart is one of the most elegant tools in the medical laboratory — simple in concept, powerful in application. It turns numerical data into a visual narrative of how our measurement process behaves over time.

But its power can only be harnessed by those who truly read the chart — not just look at it. The LJ chart provides the picture; the Westgard Multirules provide the criteria. Both work together to produce QC decisions that are objective, consistent, and accountable.