Laboratory Internal QC: Levey-Jennings Chart and Westgard Rules
What is Internal QC?
Internal quality control (IQC) monitors the stability of the testing system, including instruments, reagents, calibration and environment, by running control samples alongside patient samples every day. The Levey-Jennings chart plots daily control results around the control target as the center line, with ±1s, ±2s and ±3s control limits, so drift or trend is detected before it affects patient results. Comparing each day's result with the historical baseline is what makes early detection possible.
When to Use It
Use it for daily quality control in clinical chemistry and immunoassay, monitoring method stability over time, and supporting accreditation requirements such as ISO 15189. It is also used when introducing a new reagent lot or after instrument maintenance to confirm the system remains in control.
How to Use It (Step by Step)
Establish the control target and standard deviation from at least 20 accumulated data points, run controls with patient samples each day, and enter the results. The tool plots the Levey-Jennings chart and automatically applies Westgard multi-rules to flag violations, such as 1₃s, 2₂s, R₄s, 4₁s and 10x. On violation, troubleshoot per SOP and record the cause, correction and impact assessment on patient results.
Key Formulas / Example
The Westgard rules are: 1₃s, one point beyond 3s, reject; 2₂s, two consecutive points on the same side beyond 2s, reject; R₄s, the range within one run exceeds 4s, reject; 4₁s, four consecutive points on the same side beyond 1s, reject; 10x, ten consecutive points on the same side of the mean, reject. A common combination for routine chemistry is 1₃s/2₂s/R₄s/4₁s/10x.