A Level 2 control flags high after a reagent lot change

At 07:00 the satellite laboratory runs its assigned external controls on a nonwaived quantitative whole-blood assay. Level 1 targets 2.00 units, acceptable 1.90 to 2.10 units. Level 2 targets 8.00 units, acceptable 7.60 to 8.40 units. Both controls are pulled from refrigerator storage, treated exactly like a patient specimen, and accepted at 07:04. The analyzer's electronic internal check passes before each cycle, same as every morning.

At 10:18 a new reagent lot goes on the analyzer. The courier record for that shipment shows a documented temperature excursion in transit. At 10:25 the operator reruns controls before resuming patient testing. Level 1 reads 2.04 units, inside its range. Level 2 reads 8.63 units, above its 8.40 unit upper limit and flagged high.

The internal electronic check passed both times. Nothing about the analyzer's self-check caught this. Only the external Level 2 control, tested the same way a patient specimen is tested, caught the shift. That gap between what an internal check can see and what a full-process control can see is the reason a laboratory builds a quality control plan around where a failure can actually occur, not around running one type of check by habit.

A structured risk assessment is how a laboratory designs quality control that has a real chance of catching a failure like this one, instead of assuming the manufacturer's minimum or the last laboratory's plan will do it. A control that never fails to detect anything is not proof the process is safe; it may only mean the control was never placed where the actual risk was.