Reporting measurement uncertainty near specification limits

Accredited laboratories have been generating uncertainty estimations for quantitative testing for many years and it is now common practice when developing a new method.

While not routinely reported in disciplines such as chemistry and microbiology, ISO/IEC 17025 clause highlights the circumstance where the uncertainty of a quantitative measurement does have to be reported.

c) where applicable, a statement on the estimated uncertainty of measurement; information on uncertainty is needed in test reports when it is relevant to the validity or application of the test results, when a customer’s instruction so requires, or when the uncertainty affects compliance to a specification limit.

Many laboratory customers will not be knowledgeable of the concept of estimating measurement uncertainty. Even those that do may not be across the reporting requirements and how it may benefit them in evaluating test results. 

Compliance with ISO/IEC 17025 requires a laboratory to ensure effective communication with their customers so there is an onus on them to advise the customer about MU and to ascertain if and when they require having this reported. 

It must be noted, however, that in situations where MU is relevant to determining compliance with a specification limit, the requirements of clause c) do not accommodate a customer requesting that uncertainty not be reported.

Please note, it is not acceptable for a customer to request MU is never reported as this does not permit compliance with ISO/IEC 17025.  In discussion with customers, testing laboratories must also advise that in certain situations they are obliged to report the MU, such as when calculated results are close to the specification or cut-off limits..  Providing examples may assist in explaining this.  The laboratory should endeavour to show how MU can benefit a customer by allowing a better understanding of the results reported and hence, using this understanding in making more informed decisions.

For example if a food supplier is trying to meet a specification of <5000 cfu/g in a product and the result is 4,800 cfu/g and the MU is +/- 1000 cfu/g then the range of results within which the actual result lies will be 3,800 cfu/g to 5,800 cfu/g.  In this situation ISO/IEC 17025 requires the MU to be reported.

Similarly if we consider the specification for heterotrophic colony count in AS/NZS 3666.2 that requires action at 100,000 cfu/mL or above, a customer receiving a result of 90,000 cfu/mL where the MU is +/- 20,000 cfu/mL must also receive the MU.

In both of the given examples the customer can then evaluate the result together with the associated MU and decide how to proceed.  Failure to provide MU in these circumstances means the customer is unable to make a fully informed decision, and hence, they and consumers of their product are subject to increased risk.