Absorption of therapeutic drugs by barrier gels in serum separator blood collection tubes. Volume- and time-dependent reduction in total and free drug concentrations.

The stability of seven commonly monitored therapeutic drugs in serum was examined following storage in Vacutainer SST and Corvac serum separator blood collection tubes. Significant decreases (ranging from 5.9% to 64.5%) in the measured concentrations of phenytoin, phenobarbital, lidocaine, quinidine, and carbamazepine were observed, as a function of both time and sample volume, when serum was stored in Vacutainer SST serum separator blood collection tubes. In contrast, measured concentrations of theophylline and salicylate did not change under identical specimen storage conditions. No significant changes in the concentrations of phenytoin, phenobarbital, carbamazepine, theophylline, quinidine, and salicylate were observed when serum was stored in Corvac serum separator blood collection tubes. Only serum lidocaine concentrations decreased (ranging from 31.5% to 72.6%, depending on sample volume) after storage in Corvac tubes for 24 hours. The apparent decreases in serum concentrations of therapeutic drugs in both Vacutainer SST and Corvac tubes were most pronounced when small volumes (200-500 microL) of serum remained in contact with the barrier gels for prolonged periods of time (> 2-6 hours). These decreases were due to absorption of drugs by the barrier gels, as demonstrated by the recovery of drugs following chemical extraction of the barrier gels with methanol. For phenytoin and phenobarbital, the reduction in total drug concentrations also resulted in a proportional decrease in free drug concentrations and was dependent on the extent of protein binding by the drug. None of the therapeutic drugs used in this study were adversely affected by prolonged storage in standard red top Vacutainer blood collection tubes without barrier gels. The data suggest that serum separator blood collection tubes should be used with extreme caution for therapeutic drug monitoring, particularly when reduced sample volumes or prolonged specimen storage may be required.