ISSUES IN ACCREDITATION


Expanded (and final) uncertainty The final expanded uncertainty is derived by multiplying the combined uncertainty by the coverage factor (usually 2).


Significant figures Recommendations are to report MU results to the same number of significant figures as that reported in the measurand, with a maximum of two being used.7


Uncertainty of measurement targets Imprecision targets In the absence of a consensus target value for imprecision, a combination of locally derived targets and targets based on biological variability are commonly used. When biological variability is available and deemed appropriate, the levels of performance assessed against are (from ref 7): • Optimum: imprecision uncertainty <0.25 x biological variability


• Desirable: imprecision uncertainty <0.50 x biological variability


• Minimum: imprecision uncertainty <0.75 x biological variability


Bias targets As with imprecision uncertainty targets, the targets applied to the acceptability criteria of laboratory bias will, when available and appropriate, be set according to the biological variability of the assay according to: • Optimum: laboratory determined bias <0.125 (intra-individual biological variation2


+ inter-individual biological variation2)1,2


• Desirable: laboratory determined bias <0.250 (intra-individual biological variation2


+ inter-individual biological variation2)1,2


• Minimum: laboratory determined bias <0.375 (intra-individual biological variation2


+ inter-individual biological variation2)1,2


When not available, local laboratory acceptability criteria must be set on an individual assay basis.


Conclusions There is much discussion regarding whether or not MU will be of benefit to laboratory scientists now and in the future. It would appear that its purpose is two-fold. When used in combination with biological variation and analytical bias, MU can imply to, or reassure, a clinician that there are significant differences, or changes, in a patient’s results. From a laboratory perspective, MU will probably be used more effectively as a quality indicator, allowing us to monitor the performance of our assays.


References 1 Westgard JO, Westgard SA. The quality of laboratory testing today: an assessment of sigma metrics for analytic quality using


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Table 2. The types of error present in laboratory assays, the sources of which must be eliminated if possible. If not possible, it must be incorporated into the overall MU calculation.


Error type Description


Stochastic/random error Associated with an individual measurement. Determination of uncertainty of measurement is important to prevent over-estimation of stochastic error. This type of error is untraceable and cannot be eradicated as it is present in all measurements, but can be minimised.


Residual error


The difference between the observed value and estimated predicted value and in the haematology setting is often applied to extrapolation of sample results from reference/calibration curves.


Leverage/influential error A single point influencing the position of the mean values (for the calibration from which expected values are extrapolated) can be accounted for using studentised residuals.


Gross error/outliers


Systematic error Constant error


Proportional error Sampling error


Exert excessive influence on descriptive statistics, in particular the mean and standard deviation.


Often termed assay bias.


The absolute magnitude of the error is constant but the relative error (expressed as a percentage) changes, either increasing or decreasing as the measurand magnitude increases.


The absolute magnitude of the error changes as the value of the measurand changes (may increase or decrease) and is reflected by a constant percentage error.


Caused by the deviation of the observed value from that in the true population.


performance data from proficiency testing surveys and the CLIA criteria for acceptable performance. Am J Clin Pathol 2006; 125 (3): 343–54.


2 Joint Committee for Guides in Metrology. Evaluation of measurement data – Guide to the expression of uncertainty in measurement. JCGM100:2008.


3 Joint Committee for Guides in Metrology. Evaluation of measurement data – Supplement 2 to the Guide to the expression of uncertainty in measurement – Extension to any number of output quantities. JCGM 102:2011.


4 Joint Committee for Guides in Metrology. Evaluation of measurement data – The role of measurement uncertainty in conformity assessment. JCGM 106:2012.


5 Birch K. Estimating uncertainties in testing – An intermediate guide to estimating and reporting uncertainty of measurement in testing measurement. Good Practice Guide No. 36. British Measurement and Testing Association, 2001


6 United Kingdom Accreditation Service. The expression of uncertainty and confidence in measurement. M3003 Edition 3. UKAS, November 2012.


7 Singapore Accreditation Council. A guide on measurement uncertainty in medical testing. Technical Guide 4. SAC, February 2013.


8 White GH, Farance I; AACB Uncertainty of Measurement Working Group. Uncertainty of measurement in quantitative medical testing. Clin Biochem Rev 2004; 25 (4): S1–24.


9 European Federation of National Associations of Measurement, Testing and Analytical Laboratories. Technical Report No 1/2006, August 2006.


10 Meijer P, de Maat MP, Kluft C, Haverkate F,


van Houwelingen HC. Long-term analytical performance of hemostasis field methods as assessed by evaluation of the results of an external quality assessment program for antithrombin. Clin Chem 2002; 48 (7): 1011–5.


Useful websites • American National Standards Institute (ANSI: www.ansi.org)


• American Society for Testing and Materials (ASTM: www.astm.org)


• Eurachem (www.eurachem.bam.de) • European Cooperation for Laboratory Accreditation (EA: www.european- accreditation.org)


• International Laboratory Accreditation Cooperation (ILAC: www.ilac.org)


• International Organization for Standardization (ISO: www.iso.ch)


• International vocabulary of basic and general terms in metrology (VIM: www.cornnet.nl/ ~mlbroens/vim.htm)


• National Conference of Standards Laboratories International (NCSLI: www.ncslinternational.org)


• National Institute of Standards and Technology (NIST: www.nist.gov)


• NIST-SEMATECH Engineering Statistics Internet Handbook (www.itl.nist.gov/div898/ handbook/index.htm)


• Uncertainty Analysis (www.itl.nist.gov/div898/ handbook/mpc/section5/mpc5.htm)


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


Stephen G MacDonald, Specialist Haemostasis Unit, Pathology Partnership, Cambridge University Hospitals NHS Trust, Cambridge.


AUGUST 2016 THE BIOMEDICAL SCIENTIST


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