IQC Best Practices for NABL Accredited Medical Labs

Internal Quality Control (IQC) is the foundation of analytical quality assurance in medical laboratories. ISO 15189:2022 and NABL require a systematic, documented IQC program for all quantitative tests.

IQC is the process of testing control materials with known target values alongside patient samples. When IQC fails, it signals that something has changed in the analytical system — and patient results should not be reported until the problem is resolved.

Westgard Rules — The Standard for IQC Interpretation

Westgard rules are a series of statistical decision rules used to interpret IQC data. The most commonly applied rules are:

• 1₂s warning rule: One control outside 2 SD — warning, investigate
• 1₃s reject rule: One control outside 3 SD — reject run
• 2₂s reject rule: Two consecutive controls outside 2 SD in same direction — reject run
• R₄s reject rule: Range between two controls exceeds 4 SD — reject run
• 4₁s reject rule: Four consecutive controls outside 1 SD in same direction — reject run
• 10ₓ reject rule: Ten consecutive controls on same side of mean — reject run

Levey-Jennings Charts

All IQC data must be plotted on Levey-Jennings charts, which graphically display control results over time against warning and rejection limits. These charts must be maintained and reviewed regularly by the lab director or quality officer.

IQC Failure Response

Every IQC failure must be documented with:

1. Date and time of failure
2. Which rule was violated
3. Immediate action taken (repeat test, check calibration, check reagent, etc.)
4. Root cause investigation
5. Corrective action taken
6. Verification that the system is back in control before releasing patient results