Quality Control, Calibration, and Proficiency Testing
Core concepts
Quality control detects analytical error through control materials with known target values. Understand the difference between calibration (establishing the relationship between signal and concentration) and calibration verification (confirming that relationship over time). Know Levey-Jennings charts and Westgard rules.
Westgard rules
Recall 1-2s (warning), 1-3s (rejection), 2-2s (rejection across two levels), R-4s (range), 4-1s, and 10x. Know that random error tends to trigger 1-3s and R-4s, while systematic error tends to trigger 2-2s, 4-1s, and 10x.
Proficiency testing
Understand that PT evaluates the laboratory against peers, must be handled like patient samples, and must not be shared between laboratories. Know the difference between PT and alternative performance assessment.
Traps in questions
- Treating 1-2s as a rejection rule.
- Confusing random and systematic error patterns.
- Assuming PT results can be discussed with other laboratories.
- Forgetting that calibration must be verified after major maintenance.
Worked rule of thumb
When a control result exceeds 3 SD, reject the run and investigate before reporting patient results. When two consecutive levels exceed 2 SD on the same side, suspect systematic error and check calibration, reagents, and instrument settings.
Study guidance
Practice mapping each Westgard rule to the error type it detects and the corrective action it suggests.