When a recalibration is reversed within days: what it means for QC and patient results

A calibration that moves the factor by more than 5% and is then reversed by a second calibration within two weeks is a recognisable pattern. The first calibration was almost always wrong, and the laboratory reported patient results with the wrong factor until the second one.

How to recognise it

Plot the calibration factor over time next to the QC Levey-Jennings chart. An overshoot looks like a step up (or down) followed by a step back. The QC after the first calibration is often still "in control" because the laboratory re-established the control mean at the same time, which hides the shift. The EQAS result for that period shows the bias.

Usual causes

• Calibrator reconstituted with the wrong volume or not fully dissolved.
• Wrong calibrator lot or value sheet selected in the analyzer.
• Calibrator past its open-vial stability.
• Carry-over or a bubble in the first calibration run.
• Calibrating in response to a QC failure that was really a control-material problem.

What to do

1. Record the second calibration with the reason "calibration verification" and the finding.
2. Identify the patient results reported between the two calibrations for that analyte and assess the clinical impact of the bias (ISO 15189:2022 clause 7.5, nonconforming work; clause 7.4, release of results).
3. Open a CAPA; the root cause is in the list above far more often than in the instrument.
4. Before any future "QC failure" calibration, repeat the control from a fresh vial first.