Anita Joshi Updated: August 28, 2023
Manufacturing medical devices is a highly complex process, and calibration requirements according to ISO 13485, Clause 7.6 (Control of monitoring and measuring equipment), mean high precision and close monitoring. Accuracy of all instruments decays with usage and wear and tear. Factors such as electrical and mechanical shocks or environmental conditions like temperature and humidity may affect the accuracy of measurement. Therefore, a corrective procedure is required to maintain accuracy. In this article, learn more about ISO 13485 calibration requirements and calibration standards.
Two main ways of performing calibration:
- Calibration by comparison with a source of known value, the so-called standard.
- Calibration by comparing the measurement of the test instrument with the measurement from the calibrated reference instrument, the so-called calibrator.
What is calibration?
Calibration of instruments is one of the primary processes used to maintain instrument accuracy. The process of calibration involves configuring an instrument to provide sample measurement results within an acceptable range. This activity requires that a comparison is made between a known reference measurement (the standard equipment), and the measurement using your instrument (test instrument). As a rule of thumb, the accuracy of the calibration standard should be 10 times the accuracy of the measuring device being tested. However, an accuracy ratio of 3:1 is acceptable by most certification bodies.
The instrument used as a reference should be traceable to an instrument that is calibrated to your country’s National Standard; for example, the UK National Standard (UKAS) or the US National Institute of Standards and Technology (NIST). When no such calibration standards exist, the basis used for calibration or verification should be recorded. Calibration is usually followed by an adjustment made to the test instrument so that the output matches that of the standard.
Two important parameters that should be defined for every calibration process are equipment tolerance and the operating range of that instrument. Calibration tolerance is defined as the maximum allowable deviation between a standard of known accuracy and your test instrument. If your equipment exceeds the specified tolerance inaccuracies, it is usually adjusted.
What do you achieve by calibrating measuring and monitoring equipment?
- Restoring the accuracy of the instrument
- Adjusting or repairing an instrument that is out of calibration
- Minimizing uncertainty or error
- Ensuring the reliability and consistency of the instrument
- Keeping measurements within specification limits
- Building trust, confidence, and reliability in measurements
- Establishing traceability of the measurement to a National / International Standard, which is a mandatory requirement for most standards.
To summarize, calibration quantifies and controls errors and uncertainties within measurement processes and brings them to an acceptable level.
Does all equipment need to be calibrated?
Calibration is required for all instruments, i.e., equipment used in the production of a medical device, that may affect the characteristics of the medical product (dimensions, performance (e.g., volume measurement of spoons for administering antibiotics), or safety (e.g., temperature and humidity in storage)).
Given that manufacturers of medical devices generally do not have the necessary experts or equipment to carry out calibration, this process is usually carried out in accredited laboratories – national metrology institutions or calibration labs. It is the manufacturer’s duty to keep up with the expiration date of the calibration certificate for each piece of equipment, and to initiate the re-calibration process in a timely manner.
Each country or area has its own rules regarding calibration and frequency. In Europe, there is an agency called the European Association of National Metrology Institutes that defines the rules for the way the calibration is carried out and its frequency for each measuring device. Accreditation laboratories in Europe must comply with these guidelines, and it is best to contact them when defining the frequency of re-calibration.
What determines the frequency of calibration?
The frequency of calibration is influenced by several factors:
- In-house or external calibration program
- Usage of the instrument
- Behavior of the instrument – frequent out-of-tolerance results
- Accuracy and precision requirements
- Environmental conditions
- Overall calibration program and policy
- Instrument manufacturer’s recommended calibration interval
- Unscheduled calibration due to accidental dropping, or mishandling that leads to non-conforming results
In general, the more often the measuring device is used, the more often it will need to be calibrated. However, frequency also depends on the following factors:
- How frequently the manufacturer uses the measuring equipment (each day, each month, or less often)
- What is the accuracy acquired?
- Is the measuring equipment damaged? (Was it accidentally dropped or otherwise damaged?)
Although the manufacturer of the medical device does not carry out the calibration himself, it is his responsibility to monitor and to be in contact with the laboratory so that they always have a valid measuring instrument.
What are the types of calibration?
There are two main ways of performing calibration:
- Calibration by comparison with a source of known value, the so-called standard. Scales can be calibrated by using weights that have their own certificates. In this case, the weights are called standards, which are also subject to regular calibrations, i.e., checks.
- Calibration by comparing the measurement of the test instrument with the measurement from the calibrated reference instrument, the so-called calibrator. The thermometer and hygrometer are calibrated in this way so that the measurement results of the test instrument are compared with the results of the calibrator.
In addition, calibrations can be divided depending on the types of measurements performed by measuring equipment, into:
- Electronic calibration – equipment that measures any electrical parameters (e.g., voltmeters, transformers, AC/DC shunts)
- Mechanical calibration – equipment used to measure dimension changes in medical devices (e.g., scales, balances, calipers, height gauges)
- Pressure calibration – pressure measuring equipment (e.g., barometers, pressure gauges)
- Thermal calibration – temperature-measuring equipment
Types of calibration programs
Most companies have calibration programs that are either in-house or performed externally through a third-party calibration service provider.
- In-house calibrations are sometimes done on a daily basis, or every time the instrument must comply with a national or international standard. A documented procedure is used and records of these regular calibrations are maintained.
- Additionally, it is a common practice to get the instrument calibrated at defined intervals by a third-party calibration service provider who provides a calibration certificate from an accredited laboratory.
Practical tips for a calibration program
Here is a list of practical tips for a calibration program:
- All instruments used in the manufacturing, testing and related processes must be calibrated at all times during the life cycle of the instrument
- Design and document an SOP for calibration
- Conduct calibration training
- Create a master list of all equipment and instruments needing calibration, including details of equipment ID, make, location, etc.
- Define frequency or the intervals of calibration – weekly, monthly, quarterly, bi-annually, annually
- Define calibration range which covers the operational range of the instrument
- Design a Calibration Plan with dates and timelines for performing calibration
- Implement the program
- Monitor and maintain all records of calibration and verification, making them easily available at point of use
- Plan what is to be done in case of deviations
- Affix calibration status labels which identify date and due date of calibration, providing a control to ensure that only calibrated instruments are used
- After reviewing them carefully, store your Calibration certificates, with the process owner approving and signing them
- Once calibrated, do not adjust the instrument, as adjustments may invalidate the measurement result
- Protect equipment used in measuring and monitoring from damage and deterioration during handling, maintenance and storage
What is measurement uncertainty, and why is it important?
When the manufacturer of a medical device receives a calibration certificate from an accreditation laboratory, it is necessary to pay special attention to the part of the certificate that talks about measurement uncertainty. It is a statistical calculation of the possible error that the measuring instrument can make and still be acceptable according to the appropriate calibration standards.
It is the manufacturer’s responsibility to check how high this measurement uncertainty is and whether it affects the measurement area. For example, a medical device must be stored at a temperature of 15°C to 25°C. If a measurement uncertainty of 0.6°C is indicated on the calibration certificate for the thermometer, this means that when the thermometer shows 24.5°C, the temperature in the storage could be as high as 25.1°C, and that is no longer acceptable. Therefore, the manufacturer must incorporate this measurement uncertainty into his procedure for monitoring the temperature in the warehouse.
Compliance with ISO 13485 calibration requirements
A well-designed ISO 13485 calibration program as described above helps you maintain the accuracy of your instruments. It also helps you achieve compliance with the ISO 13485 calibration requirements: Clause 7.6 (Control of monitoring and measuring equipment).
Learn more about requirements in ISO 13485 in this free download: Clause-by-clause explanation of ISO 13485:2016.
