THE PLASMA DILUTION FACTOR - PREDICTING HOW CONCENTRATIONS IN PLASMA AND SERUM ARE AFFECTED BY BLOOD-VOLUME VARIATIONS AND BLOOD-LOSS

To determine the effects of therapeutic interventions on plasma protei n concentrations, it is often desirable to rule out nonspecific effect s of hemodilution. Because red cells are restricted to the vascular sp ace, the hematocrit (Hct) is a convenient marker. At the bedside-and e ven in scientific reports-a simple ratio of Hcts (obtained before and after the change in plasma volume) is often used to ''correct'' the va lue of interest. This is incorrect, and it may introduce a sizeable er ror. A new method, the plasma dilution factor (PDF), has been mathemat ically deduced. It accounts for the influence of any blood loss, plasm a osmolality changes, and blood volume variations on plasma and serum concentrations. In an in vitro experiment, blood loss and osmolality a nd blood volume changes were simulated through the withdrawal of vario us volumes of blood, which were replaced with smaller, identical, or l arger volumes of hypotonic, isotonic, or hypertonic solutions. The PDF accurately predicted changes in concentrations of albumin, fibrinogen , and antithrombin III. In contrast, the Hct ratio significantly under estimated the effects of dilution. Von Willebrand factor concentration s after hemodilution through dextran infusion in volunteers were the s ame as predicted by the PDF. In patients undergoing orthopedic surgery who were also given dextran, the postdilution von Willebrand factor c oncentrations were higher than predicted by the PDF. The Hct gave a fa lse impression of a decrease in the volunteers that was not explained by hemodilution, and it failed to detect the von Willebrand factor res ponse to trauma in the surgical patients. In conclusion, to account fo r unspecific effects of blood loss, osmolality changes, and blood volu me variations, plasma concentration changes should be compared not wit h Hct/Hct(0) but with the plasma dilution factor Hb/Hb(0) x (100 - Hct (0))/(100 - Hct).