On the Comparison of Serum and Plasma Samples in Troponin Assays

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Abstract
The figure presented by Dorizzi et al. (2) contains some conceptual and graphical errors. Because serum is the matrix of choice for the Dimension assay, this type of sample must be considered the reference sample and assumed to have negligible error. Consequently, the serum values should be used on the abscissa of bias plots, rather than the serum/plasma mean. The same, i.e., serum cTnI value, should be placed in the denominator for calculating the bias, in percentage, reported on the y axis. Furthermore, there is no correspondence between the mean bias reported in the text and that displayed in the figure. Dorizzi et al. state in the text: “… the second-generation assay had a mean bias for [heparin] plasma samples of −11.4%… ”, but Fig. 1B shows the opposite, with a mean bias of +14.5% and a scatter of +10 to + 100% higher values in seven samples having a cTnI value of ∼1.0 μg/L, the clinically most important concentrations. Perhaps the authors inverted the factors (plasma − serum values in the text and serum − plasma values in the figure), but they did so without any reason, and this fact markedly clouds the data presentation. Finally, the same scale should be used for the x axis in all of the graphs to permit a direct comparison of the results obtained with the two assay generations.