Levey–Jennings chart and Westgard rules

Principle

• Control materials with known expected values are tested alongside patient samples.
• The Levey–Jennings chart plots each control result against time, with lines for the mean and ±1, ±2 and ±3 standard deviations (SD).
• Westgard multirules decide whether a run is accepted or rejected, detecting random and systematic error.

Reagents and equipment

• Control materials at two or more levels (e.g. normal and abnormal), laboratory information or QC software.

Procedure

• Establish the lab's own mean and SD for each new control lot from at least 20 results over at least 20 separate runs/days, overlapping with the old lot.
• Coefficient of variation (CV) = (SD ÷ mean) × 100.
• Run controls at the frequency set by regulation and policy (e.g. at least two levels each day of testing for many chemistry tests).
• Plot results and apply the rules.

Interpretation

• 1-2s: one control beyond ±2 SD; warning to check other rules; used alone as a rejection rule it causes many false rejections.
• 1-3s: one control beyond ±3 SD; reject, usually random error.
• 2-2s: two consecutive controls beyond the same 2 SD limit (same level across runs, or two levels in one run); reject, systematic error.
• R-4s: within one run, one control beyond +2 SD and another beyond −2 SD (range over 4 SD); reject, random error.
• 4-1s: four consecutive controls beyond the same 1 SD limit; reject, systematic error.
• 10-x: ten consecutive controls on the same side of the mean; reject, systematic error.
• Shift: sudden change in control level (e.g. new reagent lot, recalibration). Trend: gradual drift in one direction (e.g. deteriorating reagent, aging lamp).

Quality control and pitfalls

• Do not release patient results from a rejected run.
• Find and correct the cause, rerun controls, and re-test patient samples back to the last acceptable run as needed.
• Simply repeating controls until they fall inside limits hides real problems.
• Document all QC actions; review charts regularly and take part in external quality assessment (proficiency testing).

Clinical use

• Core of internal quality control for quantitative tests, ensuring reliable patient results.