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 are predicted on the basis of a mathematical model. The model describes the kinetics of solute metabolism, breakdown, and interphase distribution (i.e., partitioning and exchange between formed elements and plasma) within a blood sample during sample withdrawal and storage. The model is applied to the determination of the integral over time of solute concentration in the plasma (area-under-the-curve, or AUC) from a sample withdrawn through an arterial or venous catheter. Errors in AUC determination can be substantial and are strongly dependent on the duration of sampling (T), the rate constants for solute degradation processes, the rate constant for solute exchange between the formed elements and the plasma (ke), and the equilibrium ratio for distribution of the solute between formed elements and plasma (R). When the value of the dimensionless group keT/R is small, little solute exchanges between plasma water and formed elements before the two phases of the blood are separated. When keT/R is large, the solute distribution is close to equilibrium at all times. In these two keT/R limits, the contribution of solute redistribution to sampling error is small. Sizable errors resulting from redistribution are associated with intermediate values of keT/R, even in the absence of metabolism and despite rapid separation of the phases at the end of the withdrawal period. Chemical conversion within either of the blood phases introduces additional sampling error under most circumstances.