MEASUREMENT UNCERTAINTY
Measurement uncertainty in results with no numbers: an overview
In this sixth article in his series, Stephen MacDonald looks at the widespread use of semi-quantitative assays where more research and discussion is needed into how to apply statistical approaches to their measurements, and the impact that has on clinical decision-making.
Most attention, not least in this series of articles, has been focused on quantifying measurement uncertainty in quantitative analysis. The same level of focus has not been extended to qualitative results. International guidelines often concentrate on quantitative results, avoiding the challenges of qualitative results. However, the assessment of measurement uncertainty in qualitative results is equally important. It defines incomplete knowledge within the test result. This highlights the need for further research, standardisation, and integration of measurement uncertainty assessment in qualitative analysis guidelines and practices.
Although results expressed on a ratio scale represent most results in laboratory medicine, qualitative results expressed on nominal and ordinal scales are also common. Results in cellular pathology, transfusion, immunology and microbiology are commonly qualitative. While there is a rapid development of quantitative measurement methods that replace qualitative methods in these disciplines, there is also a strong development of more qualitative methods that are highly useful for self- diagnosis and monitoring.
A distinction should be made between qualitative and semi-quantitative assays. Semi-quantitative assays use
Semi-quantitative assays use measurements to produce numerical values, which are then used to generate qualitative results.
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measurements to produce numerical values, which are then used to generate qualitative results. Measurements produce signals; only given meaning when they are compared to something. This is the
purpose of calibrating, and ultimately metrological traceability. The result classifies measurements into one of often two, but potentially more, categories. Categories may be positive vs. negative, detected vs. not detected, and many more. This is different from results not derived from a measurement but from direct observations – qualitative assays. In the absence of measurement, MU cannot be determined – it just doesn’t make sense. Sometimes the differentiation is not so clear though, and many assays that would traditionally be considered as qualitative are in fact semi-quantitative. ISO/TS 201914:2019 recognises now that qualitative results can be derived
Katherine Stember CC BY 4.0 Wikimedia Commons
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