The influence of blood collection technique on serum and plasma protein binding of disopyramide.

Serum and plasma disopyramide (D) protein binding was compared after blood was collected from four normal subjects in various Vacutainer tubes. The fraction of disopyramide bound to proteins in control serum and plasma was drug concentration dependent and correlated well with the capacity factor (N) associated with a high affinity protein binding site. D free fraction increased 60% at a postequilibrium concentration of 2 micrograms/ml in plasma following exposure of blood to green-top Vacutainer stoppers due to a 60% reduction in the affinity constant associated with the high affinity protein binding site. Heparin and EDTA had no effect on the plasma protein binding of D. These results suggest a competitive inhibition of disopyramide binding to alpha 1-acid-glycoprotein following contact of blood with rubber Vacutainer stoppers.