Out of tolerance: what to do when a calibration fails

An out-of-tolerance (OOT) calibration result means that, when the instrument arrived for calibration, its error at one or more test points was outside the specified limits. The adjustment that usually follows fixes the instrument, not the measurements you made with it. So you assess and record whether the results since the last good calibration were affected, and act on the instrument and on any affected product according to risk (ISO 13485 clause 7.6, as audited under MDSAP; ISO/IEC 17025 clause 7.10 for labs). This guide walks through that assessment in seven steps, with a worked example, an annotated report and a printable template.

What “out of tolerance” means (and what it doesn’t)

A calibration compares the instrument with reference standards and reports the error at each test point. If an as-found error (before any adjustment) is beyond the limits that apply, the instrument was out of tolerance. In the terms of the decision-rule guidance, the item “failed”: at least one test point gave a value beyond the tolerance (Global ACI-TECH-1-002, formerly ILAC G8, Appendix B, Example 1). The limits can be the manufacturer’s specification, a legal maximum permissible error (VIM 4.26, quoted in ILAC G24, §3.13), or a tolerance you set for your own use.

The phrase is used for several different things, and search results mix them up:

This guide covers the first one: a measuring instrument that failed calibration, and what that means for everything it measured.

The out-of-tolerance process at a glance

Every framework that addresses OOT results asks for the same core: evaluate the effect on previous results, record it, and act on the equipment and on affected product (MDSAP Audit Approach, Tasks 13–14; ISO/IEC 17025 clause 7.10). The flow below turns that into seven steps. Two of them can end the process early, but only with a written rationale.

Step 1: confirm it is a real out-of-tolerance

Before you open a long investigation, spend ten minutes on the certificate. Three questions decide whether this is an out-of-tolerance event for your use:

1. Which tolerance was used? Labs usually check the manufacturer’s specification. If your own tolerance for the instrument is wider or narrower, compare the as-found errors with yours.
2. What decision rule, and how large is the uncertainty? Whether a result is in or out depends on the decision rule (TECH-1-002, §5.2.3). Under a non-binary rule, a result just beyond the limit but within the guard band is a conditional fail, and close to the limit the chance that the instrument was actually in tolerance can approach 50 % (TECH-1-002, §5.2.3 and Appendix B, Example 2). A result beyond the limit by more than its expanded uncertainty is a clear fail.
3. Did it fail where you use it? An error at 300 psi matters for tests done near 300 psi. If you only use the gauge from 0 to 100 psi and every point there passed, record that, with the evidence of how the gauge is used.

A conditional fail is not a free pass. It means the evidence is uncertain, and the risk of the decision falls on whoever receives it (TECH-1-002, §1.2). When a lab reports an item as failed, it is the customer who has to investigate the impact on its products, which can lead to costly recalls (TECH-1-002, §6.4). That is a good reason to agree the decision rule with the lab before the calibration, as explained in our guide to reading a calibration certificate.

Step 2: contain the instrument and suspect product

Stop the instrument from being used, and stop suspect product from moving:

• Pull and label the instrument. Labs accredited to ISO/IEC 17025 must take equipment that is defective or outside specified requirements out of service, and isolate or clearly label it until it is shown to work correctly (clause 6.4.9, summarized by PJLA). Drug manufacturers must not use instruments that fail to meet their specifications (21 CFR 211.160(b)(4)), and FDA’s API guidance says the same (ICH Q7, §5.34).
• Hold product still in house that was measured or accepted with the instrument since its last good calibration. You can release it later if the assessment shows no impact.
• Check what else depends on it. If the instrument was used to calibrate or check other instruments, those results are suspect too.

Step 3: set the look-back window

This is the question most people ask first: how far back do I have to look? The answer is to the last time you know the instrument was good. FDA’s guidance for active pharmaceutical ingredients says that when a critical instrument deviates from its calibration standards, you should investigate whether the deviation could have affected product made since the last successful calibration (ICH Q7, §5.35). That usually means the date of the previous calibration with in-tolerance as-left results.

The end of the window is the last use before the instrument was taken out of service.

Without other data, you cannot tell when the instrument drifted out: it might have happened right after the previous calibration or just before this one (PJLA, 17025 §7.10 webinar). Intermediate checks change that. A documented check that passed, at the points you use, is evidence that the instrument was good on that date, so the window can usually start there instead (ISO/IEC 17025 clause 6.4.10; ILAC G24, §4.9–4.10).

Step 4: trace where the instrument was used

The window tells you when. Now you need where: every product, lot, test, inspection and calibration in which the instrument was used during the window. MDSAP auditors check that the effect of an out-of-tolerance condition was evaluated for devices in process, finished and already released, based on risk (MDSAP Audit Approach, Task 13).

This is where most investigations get stuck. Practitioners on r/Metrology describe users with decades in the business who have no idea what a failed instrument touched. Typical sources to rebuild the list:

• Inspection and test records that name the instrument’s asset ID.
• Check-out logs for shared gauges, and the instrument’s location.
• Calibration records of other instruments where it served as the reference.
• Batch, lot or traveler records for the processes it supported.

If you can’t link the instrument to specific results, the conservative assumption is that every result in the window could be affected. That is a strong argument for recording the asset ID on every inspection record. See our guide to calibration records.

Step 5: assess the impact (a worked example)

For each use, ask one question: knowing the as-found error, could any result you accepted have been outside its limits? The simplest method corrects past readings for the error and compares them again with the product limits:

1. Take the as-found error at the test point closest to where the instrument was used, with its sign. Error = reading − reference, so a gauge that reads high gives readings that are higher than the true value.
2. Correct each recorded result: true value ≈ recorded value − error.
3. Compare the corrected results with the product limits, keeping a margin for the calibration uncertainty.
4. Flag every result that could have crossed a limit.

This assumes the full as-found error existed throughout the window, which is conservative if the instrument drifted gradually. It is our worked illustration, not a method prescribed by a standard.

Example. Let’s continue with the digital pressure gauge from our example calibration certificate. Its as-found error was +0.90 psi at 300 psi (tolerance ±0.75 psi, U = 0.12 psi) and +0.18 psi at 75 psi. In the 12-month window, it was used for two tests:

In use A, a valve recorded at 297.8 psi passed at the time, but its corrected value is 296.9 psi, below the lower limit. Allowing for the 0.12 psi uncertainty, any valve recorded below 297.0 + 0.12 + 0.90 = 298.02 psi is suspect. In this fictitious example, that is 31 valves.

A quantitative method for higher stakes. NASA’s measurement decision risk handbook includes a “calibration feedback analysis”. For each item tested with the out-of-tolerance instrument, it estimates the probability that the item was accepted while actually out of tolerance (the false accept risk), using the instrument’s estimated bias, the uncertainty of that bias projected back to the test date, the value measured on the item, the test process uncertainty and the item’s limits. The result supports a choice between recalling the item, notifying its user, or taking no action (NASA-HDBK-8739.19-4, Appendix G). In NASA’s worked example, a multimeter found 2.81 mV below nominal, against a ±2.5 mV tolerance, gives a 10.1 % false accept risk for a DVD player attribute tested with it 370 days earlier (Table G-2).

Step 6: decide, act and notify

With the impact analysis done, each group of suspect results needs a disposition:

A few rules apply whatever your sector:

• A recall is a possible outcome, not the default. ISO/IEC 17025 asks labs to notify customers and recall work “where necessary” (clause 7.10.1, as summarized by PJLA and IAEA), and NASA’s method explicitly includes “no action” (Appendix G).
• Shipped product is in scope. Drug manufacturers must investigate unexplained discrepancies even when the batch has already been distributed, and extend the investigation to other batches that may be affected (21 CFR 211.192). For medical devices, the MDSAP audit looks at released devices too (Task 13). If affected devices have shipped, the decision feeds your nonconformance, correction and removal process.
• Write down who decided and why. In many companies, quality provides the data and engineering and management make the call on shipped product. When you lack data, give the customer the size and direction of the error and the affected serial numbers so they can assess their own risk.

Step 7: close out (root cause, interval and records)

Containing the event isn’t the end. Three things prevent the next one:

• Root cause. Drift, damage, overload, a dropped gauge, a method problem. MDSAP auditors review adjustment, calibration and maintenance records to see whether inadequate procedures, or not following them, contributed to the problem (Task 13). ISO/IEC 17025 asks labs to take corrective action when a nonconformity could recur (clause 7.10.3).
• Interval review. An OOT result is evidence that the interval may be too long. In ILAC’s “staircase” method, the interval is shortened whenever a calibration finds the deviation outside a set fraction of the maximum permissible error. ILAC warns that a very long interval risks having to withdraw many results already reported (ILAC G24, §6.2). Adjustments to intervals should be based on documented technical and statistical analysis, and NIST lists lack of funding or loss of competent staff among the inadequate reasons for adjusting one (NIST GMP 11). See our guide to calibration intervals.
• Intermediate checks. A periodic check at the critical points shortens the next look-back window from months to weeks (Step 3).

Finally, update the equipment record with the as-found values. For A2LA-accredited calibration labs, equipment records must include the measured value of every parameter found out of tolerance (A2LA I112).

When you are the calibration lab

The same logic applies inside an accredited calibration lab, with one difference: the affected “product” is your customers’ calibrations. When a reference standard is found out of tolerance, ISO/IEC 17025 treats it as nonconforming work (clause 6.4.9 leads into 7.10). The lab’s procedure must:

• define who is responsible and who can stop work or hold reports, based on the lab’s risk levels;
• evaluate how significant the nonconformity is, with an analysis of its impact on earlier results;
• decide whether the work is acceptable;
• notify customers and recall work where necessary;
• define who authorizes resuming work;

and the lab must keep records of these steps (clauses 7.10.1–7.10.2, as summarized by PJLA and IAEA).

PJLA, an accreditation body, puts it simply: the as-found condition is the condition in which the standard was used. The lab traces which customer calibrations and statements of conformity relied on the standard during the window (“reverse traceability”) and keeps a record of the investigation (PJLA, 17025 §7.10 webinar). One standard can sit under hundreds of customer certificates, so the cascade can be large.

Out-of-tolerance report: annotated example and template

An OOT report (also called an out-of-tolerance investigation or impact assessment record) is the evidence auditors ask for. The example below records the pressure gauge case from Step 5. It is fictitious: the company, lots and results are invented. Select a number to jump to its note.

Use the template below for your own investigations. Print it, or copy the fields into your quality system.

What each standard requires

Requirements differ by sector, but they converge on the same idea. For paid standards we describe requirements in our own words, based on the official guidance cited.

For the measuring-resources clause in depth, see our page on ISO 9001 clause 7.1.5.

Common myths

Next steps: check how much margin your calibrations leave with the TUR calculator, and see how out-of-tolerance handling fits into a complete calibration program.