QC passed. The patient results moved anyway.

A new lot of glucose reagent goes into service on the core chemistry analyzer. Both control levels run acceptable on the new lot before the technologist releases it: the low control reads within its target range and the high control reads within its target range. Everything on the QC log looks routine.

Two days later, a delta check flags a diabetic patient whose fasting glucose jumped from 118 mg/dL to 128 mg/dL between visits with no new insulin order and no documented illness. The tech pulls the prior specimen's lot number and the current one. They are different lots. Nothing on the QC record explains the jump, because QC never flagged anything.

The bench question here is whether a new reagent, calibrator, control, or consumable lot will produce patient results comparable enough for its intended use, and how you know before it affects a treatment decision, not after. Acceptable QC on a new lot does not answer that question by itself, because QC material can respond to a lot change differently than a patient's own plasma does. That gap between QC performance and patient-sample performance is called a matrix effect, and it is why a laboratory-defined, patient-sample-based lot verification protocol exists.

A clean QC log on a new lot is a necessary check, not a sufficient one. It does not by itself clear the lot for patient-sample comparability.