Out of Tolerance (OOT)

An out-of-tolerance (OOT) finding during calibration means that the instrument's measurements were inaccurate beyond acceptable limits while it was in use. This is a significant quality event because it raises questions about the validity of all measurements made with the instrument since its last known good calibration. The severity of an OOT condition depends on how far outside tolerance the readings are and how critical the measurements made with the instrument were.

When an OOT condition is found, a formal investigation process should be triggered. This typically includes determining the magnitude and direction of the error, identifying all products, processes, or test results that were measured with the instrument during the affected period, assessing whether the OOT condition could have caused any of those measurements to be wrong enough to matter, and documenting the investigation findings and any corrective actions taken.

For calibration management, OOT tracking is a key performance indicator. The OOT rate (percentage of calibrations that find instruments out of tolerance) is used to evaluate the effectiveness of calibration intervals — a high OOT rate suggests intervals are too long, while a very low rate may indicate intervals could be safely extended. Most organizations target an OOT rate of 2-5% as a balance between measurement reliability and calibration efficiency. Calibration management software should automate OOT notifications, track investigations, and support trend analysis of OOT rates by instrument type, manufacturer, and usage pattern.