A model for an uncertainty budget for preanalytical variables in clinical chemistry analyses.

BACKGROUND We sought a practical method to calculate preanalytical uncertainties. In clinical chemistry measurements, the combined preanalytical uncertainty is a function of the magnitude and probability distribution of the different uncertainty sources and the number of such sources. METHODS Results from an optimal practice for handling of the blood samples (termed the standard method) were compared with alternative methods that deviate from the standard method but are used in current practice. For categorically distributed uncertainty sources (e.g., use of different kinds of blood tubes), alternative treatments were modeled discretely using a known probability distribution for each alternative. For continuously distributed sources (e.g., clotting time), we assumed a rectangular distribution. We calculated the expectation, variance, and SD of differences between results from current practice and the standard method. We tabulated uncertainty budgets for the differences between current practice and the standard method for each uncertainty source. The expected individual biases and variances were summed to obtain the combined expected bias and variance. RESULTS The combined expected bias (SD) for glucose was -0.15 (0.130) mmol/L, with prolonged clotting time giving the greatest contribution. The combined expected bias (SD) for calcium was -0.011 (0.0182) mmol/L, for magnesium 0.006 (0.026) mmol/L, and for creatinine 0.5 (1.81) micromol/L. CONCLUSION By comparing a standard method for preanalytical sample handling to alternative methods used in current practice, and considering the distribution of alternative methods, our modeling approach allows the development of an uncertainty budget for preanalytical variables in clinical chemistry analyses.