MEASUREMENT UNCERTAINTY


By understanding and adhering to acceptable performance benchmarks, managing deviations, and effectively communicating MU, laboratories can ensure high standards of testing and patient care


This understanding helps in effectively communicating MU to healthcare providers and patients.


n Clear Communication: Use understandable language when reporting MU to non-specialist audiences to ensure clarity and transparency.


n Keeping Updated: Stay informed about updates in MU standards and best practices through continuous education and training.


n When is MU reported to us in the laboratory? The MU should be tailored to the diagnostic, prognostic, monitoring, and therapeutic purposes of the test. Additionally, the traceability framework and the role of IVD manufacturers in assigning MU to calibrators are crucial. However, it is noted that few IVD manufacturers currently provide this information, which should be mandatory for evaluating tenders.


Standards of reporting measurement uncertainty: Significant figures and notation When reporting MU in medical laboratories, adhering to expected standards regarding significant figures and notation is crucial for clarity, precision and consistency. These standards ensure that the uncertainty estimates are both accurate and useful for clinical decision- making.


n Significant figures in reporting MU The number of significant figures used in reporting MU should reflect the precision of the measurement and avoid implying greater accuracy than is justified. MU should generally be reported with one or two significant figures. For example: n If the result of a measurement is 5.123 units with an uncertainty of 0.045 units, it should be reported as 5.12 ± 0.05 units. Here, both the result and the uncertainty are rounded appropriately. For rounding in general the result should be rounded to the same decimal place as the uncertainty. This ensures consistency and clarity.


n If the uncertainty is calculated as 0.0567, it should be rounded to 0.06,


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and the measurement result should be rounded accordingly.


n Notation for reporting MU When reporting MU, the format and notation must be clear and consistent to avoid misinterpretation. The most widely accepted notation for reporting MU is in the form of [Result] ± [Uncertainty]. This format clearly shows the measured value and its associated uncertainty. The expectation is generally that uncertainty reports on clinical results are expanded uncertainties and use a coverage factor of 2. This gives 95(ish) percent coverage, so is not often reported. Deviation from this assumption should be noted. The assay uncertainty is generally reported as the combined uncertainty – pre expansion. Therefore, expanded uncertainty generally is only applied to clinical results. For example: n If a glucose concentration is measured as 6.3 mmol/L with an uncertainty of 0.2 mmol/L, it should be reported as 6.3 ± 0.2 mmol/L.


In some cases, MU can be expressed using parentheses, especially when the measurement result is reported with several decimal places. This is often seen in scientific publications. For example: n A result of 5.123(45) units indicates a result of 5.123 units with an uncertainty of 0.045 units. However, as this notation is less common in clinical settings and may not be as easily understood by all audiences, it should be avoided.


Always include the appropriate units with both the measurement result and the uncertainty. This avoids confusion and ensures that the reported values are meaningful in a clinical context, and of course demonstrates metrological traceability. If reporting the uncertainty of a temperature measurement, it should be presented as 37.0 ± 0.1°C rather than just 37.0 ± 0.1.


Sometimes, it may be useful to report


the relative uncertainty, especially when comparing the precision of different measurements and is best practice for some methods of calculation, including derived results. This is expressed as a


percentage of the measurement value, using the same calculation as for the coefficient of variation (CV). Again, as an example, if the result is 100 units with an uncertainty of 5 units, the relative uncertainty can be reported as 5%. If using a relative uncertainty, do not convert to a CV too early. Perform all calculations using the SD then in the final step change to a relative uncertainty.


n Best practices for reporting Documentation: Ensure that all reports, including those with MU, are well- documented within the laboratory’s quality management system. This includes clear explanations of how the MU was calculated and the significance of the reported figures. Education and Training: Laboratory staff should be trained to understand and apply these standards consistently. This helps in maintaining the accuracy and reliability of laboratory results across different tests and scenarios.


Conclusions Reviewing and reporting measurement uncertainty is integral to maintaining the accuracy and reliability of medical laboratory tests. By understanding and adhering to acceptable performance benchmarks, managing deviations, and effectively communicating MU, laboratories can ensure high standards of testing and patient care. Each stakeholder – reference measurement laboratories, local laboratories, and IVD manufacturing companies – plays a vital role in ensuring compliance with MU limits and enhancing the overall quality of laboratory services.


Further information n Clinical Laboratory Improvement


Amendments (CLIA): www.cms.gov/ Regulations-and-Guidance/Legislation/CLIA


n United Kingdom Accreditation Service (UKAS): www.ukas.com


n College of American Pathologists (CAP): www.cap.org


n Good Clinical Practice (GCP) Guidelines: https://ichgcp.net


n International Organization for Standardization. ISO 15189:2012 Medical laboratories – Requirements for quality and competence. www.iso.org/standard/56115. html


Dr Stephen MacDonald is Principal Clinical Scientist, The Specialist Haemostasis Unit, Cambridge University Hospitals NHS Foundation Trust, Cambridge Biomedical Campus, Hills Road, Cambridge CB2 0QQ.


+44 (0)1223 216746. SEPTEMBER 2024 WWW.PATHOLOGYINPRACTICE.COM


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