Post-Calibration Checks in Mapping Validations: Should you perform them?
During one of our recent webinars, we received a question regarding pre-and post-calibration checks:
"Will an FDA inspector expect to see pre- and post-calibration checks on data loggers?"
Paul Daniel's Answer:
An FDA inspector will expect to see assurance that the sensors used in mapping are accurate throughout the duration of the study. Pre-calibration checks are the standard practice to show that sensors are accurate at the start of a study. Over the years, inspectors have come to expect post-calibration checks because most people used thermocouple-based systems.
The nature of thermocouples is that normal use, such as bending, will cause the accuracy of the thermocouple to drift. If the drift has been significant, it will only be revealed by a post-calibration check. So there is a historically good reason for post-calibration checks.
Another reason to do a post-calibration check: it's always best practice to show an inspector exactly what they expect to see during an inspection! If you are using nice, stable thermistor-equipped data logger, the accuracy issues simply are not as central to the use of that measurement device. But this doesn't change the legacy left by thermocouples in GxP environments.
Here is a potential solution...
Do the post-cal check. Not to be too self-promoting, but this is REALLY easy with Vaisala data loggers. You can easily create a stable point-of-use environment. For probe-based temperature loggers, simply zip-tie the probes around your reference RTD. Then place the loggers in a small insulated container and then put the container in a controlled environment.
You can even place water bottles in the container to dampen temperature changes. If you already have this "post-cal box" stored inside the environment you are mapping, it will already be equilibrated.
This technique should easily provide an environment that is suitable for a ±0.5C single point verification. The same process could be adapted for temperature/RH loggers. If you choose to use a procedure like this, you should develop an SOP and validate the process.
Once the study is complete, put the loggers in this box overnight, leaving it in the area you just mapped (or a more stable environment if you have access to one). When you download the data for your study analysis, you have already got a data set included to use for your post-cal reports.
- Compile some data to prove that your data loggers have a minimal drift to provide you and the inspector with confidence that the data is good post-study. Our own internal statistics show that over 95% of Vaisala data loggers (over 98% for temperature channels and over 95% for humidity channels) sent to us for recalibration are still within tolerances after 1 year. (Below is a peer-reviewed paper we published on some of this data.) If you can show that you have compiled data on your own tools and present that to an inspector, it will generally make them happy.
- Make sure to include acceptance criteria that allows for a 5 - 10% failure rate on post-calibration results.
We do have customers who do not perform any post-calibration checks on loggers. Considering the data we have gathered from loggers sent to us for recalibration, we feel confident that this is a safe practice.
However, it remains to you to assess your own regulatory risk appetite. This is why we won't openly recommend skipping the post-cal. I would only recommend a customer do this if they are comfortable explaining their rationale to an auditor. It may be simpler to just show do the post-cal…
Please leave your questions/comments to me directly. I am happy to offer any clarification or guidance I can!
Senior Regulatory Compliance Expert