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TNI Standards Guidance

Disclaimer: This material represents the opinion of its authors. It is intended solely as guidance and does not include any mandatory requirements except where such requirements are referenced. This guidance does not establish expectations of being implemented universally, exclusively, in whole, or in part.

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Module: Quality Systems

Subject: Use of Measurement Uncertainty

Question 1

Isn't uncertainty always relevant to the validity of tests results?  How are labs expected to express uncertainty of tests on analytical reports?  Should it be like +/- x%? Could TNI provide guidance on calculating and expressing uncertainty?

Relevance:  Uncertainty is relevant to the validity of all test results; however, the uncertainty concept is not always accurately applied or interpreted by the data users.   Ultimately, the end users of the laboratory data are responsible for determining when the uncertainty of the analytical results are relevant for the specific project. When requested, the laboratory is responsible for being able to provide the information to the client in a manner that can be understood and utilized with respect to the data reported.

Calculations:  Measurement uncertainty in the laboratory can be determined using several procedures, including the use of laboratory quality control data, the detailed assessment of the individual steps in the procedure and the summation of uncertainty of each step, or other accepted statistical processes.  There are a vast number of resources available to assist laboratories in determining the uncertainty of data being generated, including Quantifying Uncertainty in Analytical Measurements, EURACHEM/CITAC Guide:  Second Edition, and Guide to the Expression of Uncertainty in Measurement, (GUM), ISO/IEC Guide 98- 3:2008. These are only two of the existing sources that can be used to develop a process for determining uncertainty of laboratory processes, therefore many other sources of information, including journal articles, textbooks, regulatory policy guidance, seminars/workshops, webinars, etc., on this topic are available to assist the laboratory in developing its own internal policy for making this determination in a statistically defendable manner. 

Options:  Once the laboratory has developed a policy and determined the uncertainty for each analyte in each analytical process, the relevance of measurement uncertainty on test results is defined by the client’s need and project requirements.  If the client requests that the information be included on test reports, then the uncertainty can be expressed in a number of ways.  The most accepted manner for reporting uncertainty is “X ± U at a 95% confidence limit, k=2”, where X is the sample result, U is the expanded uncertainty, and k indicates the confidence level.  The confidence level is a necessary aspect of the reporting of uncertainty data to give the data user the known degree of confidence that the determined analyte concentration will fall within the uncertainty range provided.