MEASUREMENT UNCERTAINTY


Metrological traceability: examining the way to give our results meaning


With increased awareness of metrological traceability, both from an IVD manufacturer and end-user perspective, improvements in analytical methods are possible. In this second article in his new series on measurement uncertainty, Stephen MacDonald provides a detailed overview of the issues involved.


Metrological traceability (MT) – 10 syllables in two words – can be a bit of a tongue twister but is central to all processes in the modern clinical laboratory. At its simplest it is a property of a measurement result that supports comparison and consistency. It is not a new concept; thousands of years ago uniformity of the blocks used to construct the ancient pyramids was based around master blocks and units such as the cubit. The ability of sailors to navigate the globe was largely down to the traceability of the instruments used to measure and time their passage through


Term Measurement procedure Reference measurement procedure Measurement system Definition A specification for how a measurement is performed


A procedure that provides results that are fit to assess measurement trueness measured in another procedure


An individual method that is made according to the specifications of the measurement procedure, in reference to the reference measurement procedure


Measurement method Assay


A description of the reactions within the system used to make the measurement


An often used colloquial term used to describe the measurement procedure or system


Table 1. Definitions of terms used in metrological traceability. Often some of these terms are used interchangeably, and occasionally are used incorrectly


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rarely travelled areas. For us in the laboratory a result is only traceable if it is directly, through a chain of calibrations, related to a reference, with each step contributing to the final measurement uncertainty (MU). A reference is a procedure or material that has a known and well-defined value, against which others can be compared. The procedure or material is used as the basis for calibration. That is achieved through a calibration hierarchy, allowing results between and within patients, for the same measurand, to be equivalent. This does not mean that they have to be


identical; there will always be differences between results. However, those differences are to be managed within an acceptable tolerance, minimising risk of harm to the patient.


Metrological traceability and measurement uncertainty Measurement uncertainty (combined MU) by the top-down method (ISO/TS 20914:2019) is calculated from within laboratory variability (uRW uncertainty (ucal ubias


), calibration ) when it is available, and when it is present, clinically significant


and corrected for. The propagation of uncertainty


through the traceability chain from a reference material or measurement procedure (uref


) and the final uncertainty


. The final assigned value of the calibrator uncertainty must be derived under specific measurement conditions using explicit experimental protocols. As end users this procedural information is often not available to us so determining whether the uncertainty is appropriate can be a challenge.


of the calibrator assigned value from our manufacturer combined provides ucal


How to mitigate for differences in assay results Different measurement procedures produce different results. Metrological traceability provides a way of minimising the risk of those differences being clinically significant through two often quoted, and sometimes confused, terms. The first is method harmonisation, which sometimes can be mistaken for the second – full standardisation. Harmonisation aims to provide equivalent results including reported units for a given measurand using different measurement methods. Standardisation of a result is not achieved until a documented


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