Key takeaways
- OOT is not just a calibration failure. It is a product quality event: any products measured with the out-of-tolerance instrument since its last calibration may have been accepted or rejected on the basis of incorrect measurements.
- The retrospective assessment must be documented. Even if the conclusion is "no product impact," the reasoning must be recorded. An undocumented assessment is the same as no assessment to an auditor.
- The magnitude of the deviation drives the response, a 0.1% DC voltage error on a Fluke 87V used only for maintenance electrical checks has negligible product impact; the same instrument used for final acceptance testing of electronic assemblies requires a detailed product impact assessment.
- Corrective action must address root cause, not just symptom , "sent for recalibration" is not a corrective action; "reduced calibration interval from 12 to 6 months due to observed drift rate" is.
- The return-to-service decision must be documented and authorised, who approved the instrument's return to service, after what evidence, and with what conditions.
The 8-step OOT response at a glance
Every out-of-tolerance event requires a structured, documented response. The table below outlines the eight steps, the responsible owner for each, and the record that must be produced. Details on each step follow in the sections below.
| Step | Action | Owner | Record required |
|---|---|---|---|
| 1 | Instrument quarantine | Lab / QA | Tag instrument "DO NOT USE. OOT" |
| 2 | Notify relevant departments | QA Manager | Email / system notification log |
| 3 | Identify exposure period | QA | Date of last in-tolerance calibration to OOT finding date |
| 4 | Identify affected products/processes | Production + QA | List of lots, work orders, or serial numbers measured |
| 5 | Perform impact assessment | QA Engineer | Written assessment with conclusion and rationale |
| 6 | Execute containment action | Production / QA | Rework, re-test, quarantine, or accept-with-justification |
| 7 | Determine root cause | QA / Metrology | 5-Why or Fishbone; identify why drift occurred |
| 8 | Implement corrective action | QA / Maintenance | Updated interval, new procedure, supplier change, etc. |
What "out-of-tolerance" actually means
A calibration compares an instrument's reading against a traceable reference standard. The instrument's specification defines the acceptable deviation, for example, ±0.1% of reading for DC voltage on a precision multimeter, or ±0.5°C for a reference thermometer. Out-of-tolerance means the instrument's error at one or more calibration points exceeded this specification.
The magnitude matters significantly. A 0.15% error when the specification is 0.1% is a minor overage. The instrument was slightly beyond its stated accuracy, but close. A 2% error when the specification is 0.1% is a major failure: the instrument was twenty times outside its specification, and any process decision based on its readings during the exposure period must be treated with serious scepticism.
The calibration certificate from a SAC-SINGLAS accredited laboratory will report the actual measured deviation at each calibration point alongside the specification limit. This tabulated data (not just a pass/fail stamp), is what enables a proper product impact assessment. It tells you not just that the instrument failed, but by how much and at which measurement points.
The ISO 9001 requirement in plain English
ISO 9001:2015 clause 7.1.5.2 states: "When monitoring or measuring equipment is found to be unfit for its intended purpose, the organisation shall determine if the validity of previous measurement results has been adversely affected, and shall take appropriate action as necessary."
The key phrase is "previous measurement results." Not just future measurements, past ones too. The standard is asking you to look backwards across the exposure period and assess the quality impact of measurements made with an instrument you now know was not performing to specification. This is the clause that generates the most calibration-related audit findings in Singapore manufacturing, because many organisations respond to an OOT finding by sending the instrument for recalibration and returning it to service. Without performing or documenting any retrospective assessment.
The auditor's question is direct: "You found this instrument out-of-tolerance. What did you do about the products measured with it while it was OOT?" If the answer is "we sent it back to the lab," that is not a satisfactory response under clause 7.1.5.2.
Step 1. Immediate quarantine and notification
The moment an OOT finding is returned on a calibration certificate, a sequence of actions must begin immediately, before any assessment is complete.
- Physically quarantine the instrument. Attach a red "OUT OF SERVICE, do NOT USE" tag. Remove it from the calibration ready area, the production floor, or wherever it normally resides. It must not be used for any measurement until the OOT response is complete and the instrument has been recalibrated and returned to service.
- Enter the instrument as OUT-OF-SERVICE in your calibration register. If your calibration management system supports status flags, update it immediately. If it is a paper or spreadsheet register, annotate it with the date and status.
- Notify all departments that used the instrument during the likely exposure period. Do not wait until you have completed the assessment. Departments need to know immediately so they can hold any suspect product pending the outcome.
- If the instrument was used on products currently in the field or with customers, notify quality leadership immediately. A decision on customer notification may be needed before the full assessment is complete.
Speed matters at this step. The longer the OOT instrument remains available for use (or the longer departments continue processing or shipping product without knowing), the larger the containment problem becomes.
Step 2. Determining the exposure period
The exposure period is the interval during which the instrument may have been out of tolerance: from the date of the last in-tolerance calibration to the date the OOT finding was discovered.
Example: the instrument was last calibrated twelve months ago and returned with an in-tolerance certificate. Today's calibration returns OOT. The exposure period is up to twelve months. During those twelve months, the instrument may have been OOT for any part of that period. Possibly from day one, possibly only in the final weeks before recalibration.
In most cases, you cannot determine exactly when the drift occurred. The conservative and auditor-accepted assumption is to treat the entire interval since the last in-tolerance calibration as the exposure period. If objective evidence exists (for example, a documented pre-use check on a specific date that showed the instrument within tolerance), you may be able to justify a narrower exposure period with documented rationale. Without such evidence, use the full interval.
Once the exposure period is defined, identify every use of the instrument during that period: which production lots, which work orders, which serial numbers were measured, and at what measurement points.
Step 3. Impact assessment: the critical question
The product impact assessment is the analytical core of the OOT response. For each use of the instrument during the exposure period, the central question is: "If the instrument was reading [X% or X units] out of tolerance, would the measurement result have led to a different accept/reject decision?"
This requires comparing the magnitude of the OOT deviation against the process tolerance. The specification the measured product or process must meet. If the process tolerance is much larger than the instrument error, the impact is likely negligible. If the instrument error is comparable to or larger than the process tolerance, then incorrect measurement results (and therefore incorrect quality decisions), are a real possibility.
If the conclusion is no impact: document the rationale clearly. "The instrument exhibited a maximum deviation of ±0.08°C at the failure point. The process tolerance for this measurement is ±2°C. The instrument error is therefore less than 5% of the process tolerance. No product impact is assessed." That written conclusion, signed by the QA Engineer, is what closes the assessment.
If the conclusion is potential or confirmed impact: proceed to containment actions covering affected lots. The assessment must identify which specific lots, batches, or units were potentially affected and what action is required for each.
Many OOT events in Singapore manufacturing result in a "negligible impact" conclusion. The OOT deviation is small relative to the process tolerance, or the instrument was used in a non-critical measurement role. But that conclusion still requires documentation. An auditor cannot accept "we determined there was no impact" without seeing the reasoning recorded in writing.
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Step 4. Containment actions
Containment actions depend directly on the impact assessment conclusion. The appropriate response is proportionate to the assessed risk, and it must be documented regardless of which option is taken.
No product impact assessed
Document the conclusion with rationale. Send the instrument for recalibration. Upon receipt of the in-tolerance certificate, return the instrument to service with appropriate authorisation recorded. No further containment action is required, but the full OOT response file must be retained.
Potential impact on in-process product
Quarantine all production lots, work orders, or assemblies measured with the OOT instrument during the exposure period. Perform re-inspection or re-testing of the quarantined product using a known in-tolerance instrument. Based on the re-inspection results, make accept/reject decisions and document them. Release product from quarantine only after re-inspection is complete and the results are recorded.
Impact on shipped product
Customer notification may be required. The decision depends on the nature of the product, the magnitude of the potential measurement error, the product's end use, and any contractual or regulatory obligations. Document the decision and the reasoning regardless of whether notification is issued. If notification is issued, retain records of all communications.
GMP pharmaceutical environments
Any OOT finding on a process-critical instrument in a GMP-regulated environment (temperature, pressure, humidity, balance), may trigger batch investigation requirements under HSA GMP guidelines, WHO TRS, or applicable international pharmacopoeia. The instrument's role in batch release decisions determines whether a batch review is required. Consult your Qualified Person or RP and your quality management system procedures before concluding the assessment.
Aerospace maintenance under CAAS Part 145
If the instrument was used on aircraft maintenance tasks, CAAS Part 145 may require notification to the continuing airworthiness management organisation and potentially to the aircraft owner. Review your Part 145 quality manual and approved procedures for the specific notification and investigation requirements that apply.
Step 5. Root cause analysis
The corrective action cannot be designed without first understanding why the instrument drifted out of tolerance. Root cause analysis does not need to be complex (a structured 5-Why or a Fishbone (Ishikawa) diagram is sufficient for most OOT investigations), but it must be honest and specific.
Common root causes for instrument OOT events in Singapore manufacturing include:
- Calibration interval too long for the observed drift rate. The instrument's performance degraded faster than the calibration schedule assumed. The corrective action is to shorten the interval, not just recalibrate this time.
- Instrument used outside its rated environmental range. Measurement in high-temperature, high-humidity, or vibration-prone environments can accelerate drift. The corrective action may be to select a more robust instrument or to control the measurement environment.
- Physical damage, dropped, overloaded, or mishandled. If the instrument was subjected to a shock or overload event, the damage may have caused the OOT condition. The corrective action is to implement a formal damage reporting and quarantine procedure so that future events are caught before recalibration is due.
- Instrument approaching end of useful life. Components degrade over time. If the instrument has accumulated many calibration cycles and repeated OOT events, replacement is likely the appropriate response rather than continued recalibration.
- Calibration laboratory error. Occasionally, the OOT finding is attributable to an error in the calibration itself. Reference instrument out of specification, technician error, or environmental condition at the lab. If you suspect this, contact the calibration laboratory and request a technical review. If confirmed, obtain a corrected certificate.
"We sent it for recalibration and it passed" is not a root cause. It describes what happened after the finding, not why the finding occurred. An auditor reviewing a corrective action that lacks a root cause will raise a further finding. This time against your corrective action process, not just the original OOT event.
Step 6. Corrective action and preventive action
Corrective action addresses the specific OOT event and its root cause. Preventive action extends the learning to reduce the risk of similar events across other instruments or processes. Both should be recorded in your corrective action system with a due date, an owner, and a verification step.
Examples of corrective and preventive actions that auditors find credible:
- Reduce the calibration interval for instruments with high observed drift rates. Document the basis: "instrument drifted by X% in twelve months against a specification of Y%; interval reduced to six months."
- Implement pre-use verification checks. A daily or weekly check against a reference artefact that provides a quick indication of performance between full calibrations. Record the results.
- Improve instrument storage: cases, foam protection, controlled-humidity cabinets for sensitive instruments. Address the environmental factors identified in the root cause.
- Add OOT event alerts to your calibration management system so that any OOT certificate triggers an automatic notification to the QA Manager rather than being filed without action.
- Implement a formal damage reporting procedure: technicians must report any shock, overload, or suspected damage event immediately, triggering unscheduled calibration.
- Build a lifecycle replacement schedule for instruments with known limited service lives.
- If the calibration laboratory was at fault, change supplier or implement a verification check on received certificates.
Step 7. Documentation requirements
The OOT response file is what the auditor will review. Every element must be present. A gap in any item is a potential finding in its own right. Not just evidence of a weak OOT response, but of a weak quality management system.
The complete OOT response file must contain:
- The calibration certificate showing the OOT finding, with point-by-point deviation data
- Quarantine record: date, person, instrument identified by asset number and description
- Notification record: who was notified, when, and by what means
- Exposure period determination: date of last in-tolerance calibration to OOT discovery date
- List of affected products, processes, or lots during the exposure period
- Written impact assessment: conclusion, rationale, QA Engineer's name and signature, date
- Containment actions taken: what was done, when, and by whom, with evidence (re-inspection records, quarantine tags, customer notification letters as applicable)
- Root cause analysis: 5-Why or Fishbone output, signed and dated
- Corrective and preventive action: specific actions, due dates, owners
- Evidence of closure: recalibration certificate showing in-tolerance result; re-inspection records if applicable; confirmation of preventive action implementation
- Return-to-service authorisation: who approved the instrument's return to active service, based on what evidence, and whether any conditions apply
Retain this file for the period required by your quality management system and any applicable regulatory framework. For most Singapore manufacturers, a minimum of five years is a defensible baseline. GMP pharmaceutical and medical device sectors typically require seven to fifteen years.
Step 8. Return to service
An OOT instrument cannot simply be sent for recalibration and returned to the floor when the certificate arrives. The return-to-service decision is a formal quality action that must be authorised and recorded.
Before returning the instrument to service, confirm that: (a) the recalibration certificate shows in-tolerance results at all calibration points; (b) the OOT response file is complete, including impact assessment and root cause analysis; (c) any containment actions required by the impact assessment have been completed or are under documented control; (d) the corrective action has been initiated with a defined owner and due date; and (e) the calibration register has been updated with the new calibration date, the new due date reflecting any interval adjustment, and a note of the OOT event.
The return-to-service authorisation should be documented in the OOT response file: name, role, date, and the basis for the decision. For critical instruments used in product acceptance or regulatory-required measurements, consider requiring QA Manager sign-off rather than delegating return-to-service to the technician.
Frequently asked questions
An out-of-tolerance finding occurs when a calibration reveals that an instrument's error at one or more measurement points exceeds the specification defined for that instrument. The calibration certificate will report a deviation beyond the permitted tolerance, signalling that the instrument was not performing within its stated accuracy during the period since its last in-tolerance calibration. OOT is not simply a maintenance event. It is a quality event requiring formal investigation and documentation under ISO 9001 clause 7.1.5.2.
ISO 9001:2015 clause 7.1.5.2 requires the organisation to evaluate whether previous measurement results have been adversely affected, and to take appropriate action on any product or service that may have been affected. The standard is intentionally non-prescriptive about what "appropriate action" looks like, that is risk-based and depends on the instrument's role, the magnitude of the OOT deviation, and the process tolerances involved. However, the assessment must be performed and documented; an undocumented assessment carries no weight with an auditor.
The exposure period runs from the date of the last in-tolerance calibration to the date the OOT finding was discovered. In most cases you cannot determine exactly when during that interval the instrument drifted out of tolerance. The conservative and auditor-accepted assumption is that the instrument may have been OOT since the last calibration. If there is objective evidence such as a documented pre-use check at a specific date showing the instrument within tolerance, you may be able to justify a narrower exposure period with documented rationale and QA approval.
A product impact assessment determines whether any products measured with the OOT instrument during the exposure period may have been incorrectly accepted or rejected. The depth should be proportionate to the risk: if the OOT deviation is small relative to the process tolerance and the instrument was used only for non-critical maintenance checks, a brief written conclusion with rationale may suffice. If the instrument controlled a critical product acceptance decision, a full lot-by-lot assessment with re-inspection may be required. In both cases, the conclusion and the reasoning must be written and signed.
Not automatically. Customer notification is required when the impact assessment concludes that products with potential OOT-related measurement errors have been shipped to customers, and the error could have led to acceptance of non-conforming product. If the impact assessment demonstrates no product impact (because the instrument error was within the process tolerance, or the instrument was not used on customer product), notification is generally not required. In pharmaceutical, aerospace, and other regulated industries, sector-specific rules may impose additional notification obligations regardless of the impact assessment conclusion.
Acceptable corrective actions address the root cause of the OOT event, not just the symptom. "Sent for recalibration" is not a corrective action. It restores the instrument to service without addressing why it drifted. Auditors accept actions such as: reducing the calibration interval based on the observed drift rate; implementing documented pre-use verification checks; improving instrument storage or handling to address identified environmental root causes; scheduling lifecycle replacement for aging instruments; or changing calibration suppliers where laboratory error was identified as the root cause.
ISO 9001 requires retention of documented information as evidence of conformity for a defined period. The standard does not mandate a specific minimum. In practice, records should be retained for the longer of: the period required by your quality management system, the product's design life for durable goods, and any applicable regulatory retention requirement. For most Singapore manufacturers, five years is a defensible baseline. Pharmaceutical and medical device sectors typically require seven to fifteen years. Define and document your retention period in your quality management procedures.
Unitest Instruments Pte. Ltd. is a SAC-SINGLAS accredited calibration laboratory (ISO/IEC 17025, no. LA-2023-0845-C) based in Singapore. We calibrate electrical, temperature, pressure, humidity, and related instruments for manufacturers, service providers, and regulated industries across Singapore and the region.
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