Laboratory Quality Control Practice Test

In laboratory quality control, how you respond to control-rule violations depends on whether the issue appears once or repeats in a pattern. A single deviation outside the acceptable range is often treated as a probable random error and may be issued as a warning rather than discarding the run. But when the same kind of deviation occurs again in consecutive measurements, or when you see a sequence that suggests a systematic problem (for example, two consecutive results outside 2 SD on consecutive tests, or a pattern like a +4 SD followed by another violation such as 4-2s), that signals a real issue with the method, instrument, or reagents. In those cases, you reject the run and start an investigation to identify and fix the underlying cause. Replacing the instrument is a major step that isn’t warranted from a single outlier; it’s reserved for clear, demonstrated instrument failure. So the scenario where a single 1-2s deviation might be a warning, but repeated 1-2s or 4-2s violations lead to rejection and investigation, reflects how QC practices escalate actions based on pattern and persistence of errors.