Calculation of Measurement Uncertainty in Clinical Chemistry

Abstract

Patriarca et al. (1) recently presented in this journal “a working example” for the calculation of measurement uncertainty (lead in blood) according to the rules of the ISO Guide to the Expression of Uncertainty in Measurement (GUM) (2). Although the authors presented many estimates of imprecision and trueness, they failed to correctly combine the estimates. They estimated the overall long-term SD from quality-control samples (SDcontrol) and from human samples (SDhuman) and combined them by taking the “square root of the sum of the squares”. Instead, however, they should have used the pooled SD of both estimates, to be calculated as: \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[\mathrm{SD}\_{\mathrm{pooled}} {=} {[}\frac{(\mathrm{n}\_{\mathrm{control}} {-} 1) {\times} \mathrm{SD}\_{\mathrm{control}}^{2} {+} (\mathrm{n}\_{\mathrm{human}} {-} 1) {\times} \mathrm{SD}\_{\mathrm{human}}^{2}}{(\mathrm{n}\_{\mathrm{control}} {+} \mathrm{n}_{\mathrm{human}} {-} 2)}{]}^{0.5}\] \end{document} with n the number of samples. They presented four estimates of trueness, i.e., measurement of certified reference materials (CRMs), recovery, and results from two external quality assessment surveys (EQAs) and combined them into one mean index ( R m), disregarding the fact that the estimates conflicted with each other and were dependent on the concentration of lead. Generally, one should not use second- (recovery) or third-choice estimates (EQA results with poorly defined target values) for the uncertainty calculation when one has available first-choice estimates (comparison with CRMs). Consequently, from the data presented, we would conclude that the method is bias-free in the high concentration range (∼130 μg/L), has a considerable bias in the … [↵][1]aAuthor for correspondence. Fax 39-06-49903686; e-mail marina.patriarca{at}iss.it. [1]: #xref-corresp-2-1