ESTIMATION OF BLOOD-SAMPLING ERRORS RESULTING FROM METABOLISM AND SOLUTE EXCHANGE BETWEEN PLASMA AND FORMED ELEMENTS

The origin and magnitude of potential errors in whole-blood sampling a re predicted on the basis of a mathematical model. The model describes the kinetics of solute metabolism, breakdown, and interphase distribu tion (i.e., partitioning and exchange between formed elements and plas ma) within a blood sample during sample withdrawal and storage. The mo del is applied to the determination of the integral over time of solut e concentration in the plasma (area-under-the-curve, or AUC) from a sa mple withdrawn through an arterial or venous catheter. Errors in AUC d etermination can be substantial and are strongly dependent on the dura tion of sampling (T), the rate constants for solute degradation proces ses, the rate constant for solute exchange between the formed elements and the plasma (k(e)), and the equilibrium ratio for distribution of the solute between formed elements and plasma (R). When the value of t he dimensionless group k(e)T/R is small, little solute exchanges betwe en plasma water and formed elements before the two phases of the blood are separated. When k(e)T/R is large, the solute distribution is clos e to equilibrium at all times. In these two k(e)T/R limits, the contri bution of solute redistribution to sampling error is small. Sizable er rors resulting from redistribution are associated with intermediate va lues of k(e)T/R, even in the absence of metabolism and despite rapid s eparation of the phases at the end of the withdrawal period. Chemical conversion within either of the blood phases introduces additional sam pling error under most circumstances.