Using the EMITreg; assay, we tested clinical samples from patients receiving 12 commonly monitored drugs to evaluate the effect of serum separator gel contained in serum separator blood collection tubes (SST) (Becton-Dickinson). There were significant concentration decreases for lidocaine, pentobarbital, and phenytoin. In vitro experiments demonstrated that this effect on phenytoin was dependent on time of exposure to the gel and volume of whole blood, but was not dependent on the presence of red blood cells or initial concentration. Bias attributed to the use of SST could interfere with the usefulness of clinical results at the upper and lower limits of the therapeutic range. This problem can be minimized by processing samples of at least 2 ml within 1 h.
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