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ACB-CLIN-CHEM-GEN  February 2010

ACB-CLIN-CHEM-GEN February 2010

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Subject:

Re: Measurement Uncertainty

From:

Roger Ekins <[log in to unmask]>

Reply-To:

Roger Ekins <[log in to unmask]>

Date:

Mon, 1 Feb 2010 15:44:59 +0000

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Deluged by all the statistics, I'm somewhat 
confused by the absence of any reference  in this 
discussion to the fact that the great majority of 
substances assayed by clinical chemists are 
heterogeneous in molecular composition, assays of 
which are therefore method dependent. In these 
circumstances, there is no correct result, and 
the concept of 'bias' becomes meaningless. 
Furthermore the precision of measurement of an 
analyte concentration generally varies with 
concentration, a relationship represented by the 
"precision profile". This too seems to be ignored 
by discussants.

Roger Ekins



>I have been following the discussion on 
>estimation of uncertainty with much interest. 
>However, I am concerned that the 
>interchangeability in the use of the terms total 
>error (TE), total error allowable (TEa) and 
>measurement uncertainty (U) may be confounding 
>the understanding and application of these 
>inter-related concepts.
>The concept of measurement uncertainty is meant 
>to reflect the actual variation of a measured 
>result, not its potential to be in error. While 
>it is true that any given QC procedure will 
>allow for a certain amount of error before 
>detection, this does not mean that the 
>uncertainty of a measured result is impacted to 
>the full extent of that potential error. Let me 
>illustrate my point with the following example:
>CV = 1.895%, bias = 0.1%. Suppose the QC 
>procedure used gives an SDcrit of 2.584, then
>TEa = CV(SDcrit + 1.65) + bias
>TEa = 1.895(2.584 + 1.65) + 0.1  = 8.1%
>Note that this is total error allowable, not 
>total error. Total error would be:
>TE = (CV x 1.96) + Bias
>TE = (1.895 x 1.96) + 0.1 = 3.8%
>Uncertainty (U) would be calculated (using a coverage factor of 2) as follows:
>U = CV x 2
>U = 1.895 x 2 = 3.8%
>Other contributing sources of uncertainty (such 
>as calibration uncertainty) can be included if 
>known in the estimation of uncertainty (by 
>adding in quadrature). Note that bias is not 
>included as it is assumed that any significant 
>bias is corrected and if so the uncertainty of 
>this correction can be included.
>
>Now, let us say the performance of this assay 
>changes such that the CV increases to 3.79%, 
>which increases the TE to (3.79 x 1.96) + 0.1 = 
>7.5%. The total error allowable (TEa) remains 
>the same as it is based on a CV obtained during 
>stable analytical performance. Measurement 
>uncertainty changes to 3.79 x 2 = 7.58%.
>
>One of the provisos of measurement uncertainty 
>and TE is that the variation measured must 
>reflect the real variation of the measurement 
>process across all analytical influences, not 
>just under ideal conditions. On the other hand, 
>the variation for calculation of TEa should 
>reflect stable analytical performance, which we 
>want to use to determine the control procedure 
>for the assay.
>
>Cheers
>David Parry
>Winnipeg, Canada
>
>
>
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>------ACB discussion List Information--------
>This is an open discussion list for the academic 
>and clinical community working in clinical 
>biochemistry.
>Please note, archived messages are public and 
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>are those of the individual and they are 
>responsible for all message content.
>ACB Web Site
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-- 
With best wishes,

Roger Ekins


Prof Roger Ekins,  PhD DSc FRS

Windeyer Institute
University College London
London W1T 4JF


Phone +44 20 7679 9410
Fax +44 20 7679 9407

------ACB discussion List Information--------
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