A calibration SOP defines how, when, and by whom an instrument gets calibrated — but the specific data captured at each calibration event matters as much as the schedule itself. The single most commonly missed detail: recording only the as-left reading, without the as-found reading.
As-found vs. as-left
As-found is the instrument's reading before any adjustment during calibration — it tells you whether the instrument was actually accurate during the entire period since its last calibration. As-left is the reading after adjustment. If you only record as-left, you can never demonstrate whether every test result generated on that instrument since its last calibration was actually reliable — you've only proven the instrument is correct going forward, not that it was correct in the period you're really being asked about.
Why this is a real audit finding, not a technicality
If an instrument is found significantly out-of-tolerance (as-found) during a calibration, every result generated since its previous calibration is now suspect and needs an out-of-calibration (OOC) impact assessment. Without a recorded as-found value, you have no way to know this happened.
Setting tolerance limits correctly
Tolerance shouldn't be an arbitrary number — it should be derived from three inputs: the instrument's own manufacturer specification, the measurement uncertainty of the calibration itself, and the actual precision your GMP process requires. A tolerance set looser than what your process actually needs means an instrument can be "in tolerance" while still producing results too imprecise for your real use case.
Traceability: the detail that trips up India-specific content
Much generic calibration content defaults to saying results are "NIST-traceable" — a US-centric phrase that's technically incorrect for an Indian NABL context. In India, the correct traceability terminus is the National Physical Laboratory (NPL) India, not NIST. Calibration certificates for NABL-accredited labs should show an unbroken traceability chain up through NPL India (or an equivalent recognized National Metrology Institute), per ISO/IEC 17025:2017.
A minimal, complete calibration SOP checklist
- Instrument identification and calibration frequency (risk-based, documented, not arbitrary).
- As-found reading, recorded before any adjustment.
- As-left reading, recorded after adjustment (if any was needed).
- Measurement uncertainty and applied tolerance limit.
- Traceability reference to the correct national metrology chain.
- Independent reviewer signature — never the same person who performed the calibration.
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