L-j chart for 10 marks
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Levey Jennings chart laboratory quality control Westgard rules
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Levey-Jennings (L-J) Chart
Definition:
A Levey-Jennings chart, also called a Shewhart chart, is a graphical statistical quality-control tool used in clinical laboratories. It plots sequential internal quality-control (IQC) results against time or run number to determine whether an analytical method remains under control. The central line represents the mean, and other horizontal lines show standard deviation (SD) limits. Laboratory QC text
A Levey-Jennings chart, also called a Shewhart chart, is a graphical statistical quality-control tool used in clinical laboratories. It plots sequential internal quality-control (IQC) results against time or run number to determine whether an analytical method remains under control. The central line represents the mean, and other horizontal lines show standard deviation (SD) limits. Laboratory QC text
Diagram
Control
result
↑
+3 SD ─────────────────────────────────────
+2 SD ─────────────────────────────────────
+1 SD ─────────────────────────────────────
Mean ───────────────────────────────────── Target mean
-1 SD ─────────────────────────────────────
-2 SD ─────────────────────────────────────
-3 SD ─────────────────────────────────────
1 2 3 4 5 6 7 → Day / run number
Example plot: ● ● ● ● ●
Construction
- Select a stable control material, usually at two or three concentrations: low, normal, and high.
- Analyze each new control lot repeatedly, commonly 20 times.
- Calculate:
- Mean (x̄) = sum of observations / number of observations
- Standard deviation (SD) = measure of dispersion around the mean.
- Draw the mean as the central horizontal line.
- Draw horizontal lines at +1, +2, +3 SD and -1, -2, -3 SD.
- Plot every daily or run-wise QC result on its respective chart. A separate L-J chart is maintained for each level of control.
Quick Compendium of Clinical Pathology, 5th ed., p. 436.
Interpretation
In a normally distributed stable method:
- About 68.3% of results lie within ±1 SD.
- About 95.4% lie within ±2 SD.
- About 99.7% lie within ±3 SD.
A single result within ±2 SD is generally acceptable. A result outside ±2 SD requires review under Westgard rules. Patient reports should not be released if the analytical run is judged out of control.
Henry's Clinical Diagnosis and Management by Laboratory Methods.
Henry's Clinical Diagnosis and Management by Laboratory Methods.
Westgard rules applied to L-J chart
| Rule | Meaning | Main error detected |
|---|---|---|
| 1₂s | One value exceeds ±2 SD | Warning rule, investigate |
| 1₃s | One value exceeds ±3 SD | Random error, reject run |
| 2₂s | Two consecutive results exceed 2 SD on same side | Systematic error |
| R₄s | Difference between two controls in same run exceeds 4 SD | Random error |
| 4₁s | Four consecutive results exceed 1 SD on same side | Systematic error |
| 10x | Ten consecutive results on one side of mean | Systematic error or shift |
Patterns detected
-
Random error: Scattered, unpredictable points, often detected by 1₃s or R₄s rules. Causes include pipetting error, air bubbles, poor mixing, or operator error.
-
Shift: Sudden persistent movement of QC values to one side of the mean. Causes include reagent lot change, recalibration, instrument servicing, or change in temperature.
-
Trend (drift): Gradual progressive movement upward or downward. Causes include deteriorating reagent, light-source deterioration, electrode corrosion, or accumulating debris in tubing.
Action when the chart is out of control
- Do not report patient results from the affected run.
- Check control material, expiry, storage, and preparation.
- Check reagents, calibration, instrument function, and environmental conditions.
- Repeat QC using a fresh control aliquot.
- Correct the error, recalibrate if needed, and repeat QC.
- Repeat affected patient samples before releasing results.
Uses
- Monitors precision and stability of laboratory analytical methods.
- Detects random and systematic analytical errors early.
- Assists in deciding whether patient results can be accepted and reported.
- Provides documentation of internal quality-control performance.
Summary: L-J charts are simple, practical graphical tools for daily laboratory IQC. They show whether control values are randomly distributed around the mean or show warning patterns, shifts, or trends requiring corrective action.
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