Calibration & Traceability

A calibration is only meaningful if the reference standard used is itself traceable - meaning it can be linked, through an unbroken chain of comparisons, back to a national metrology standard (NIST in the US). Each link in that chain carries its own documented uncertainty, and the whole point of traceability is that you can, in principle, follow the chain all the way back and know exactly how confident to be in the final measurement.

Your own instruments need calibration too

Your safety analyzer, gas flow analyzer, and other reference instruments need their own annual (or manufacturer-specified) calibration against a traceable standard. This is easy to overlook because the instrument keeps producing numbers regardless of whether it's actually still accurate - an out-of-calibration test instrument silently invalidates every test it performs, and unlike a patient device failing outright, it gives no obvious warning that something is wrong.

Documentation discipline

Keep calibration certificates on file and reference them in audits - "we calibrated it" without a traceable certificate doesn't satisfy most accreditation requirements, because there's no way to verify the claim after the fact. The certificate itself should show the reference standard used, the measured deviation, and the date, not just a pass/fail stamp.

Tolerance-band discipline

A device that's "close enough" on a calibration check but outside the manufacturer's stated tolerance is out of calibration, full stop - tolerance bands exist because the manufacturer validated clinical performance against them during development, and drifting outside that band means the device is now operating in territory the manufacturer never actually verified as clinically acceptable. There is no professional judgment call to make here beyond following the documented tolerance.