Evaluation of serum and rapid serum separator collection tubes for therapeutic drug assays

Appropriate specimen collection and storage is essential to preserve sample integrity and ensure accurate test results. The default collection containers for blood drug concentrations are tubes without gel separators to avoid possible drug adsorption. However, routine chemistry tests are generally collected in gel separator tubes due to their convenient transport and processing; collection of an additional gel-free tube is often required for drug measurements. Citrated whole blood was pooled, spiked with drug, and transferred to three tubes (red, SST gold, RST orange) containing calcium chloride. Blood was allowed to clot, centrifuged and stored at ambient temperature (24 h) or refrigerated (7 days). At defined times, serum drug concentrations were determined (Roche cobas c502). Based on these results, specimen collection requirements were updated to allow serum separator tubes for 17 assays. Of the 21 assays evaluated, 18 displayed acceptable stability in both gel-containing tubes (acetaminophen, amikacin, carbamazepine, digoxin, ethanol, gentamicin, lamotrigine, levetiracetam, lithium, methotrexate, phenobarbital, phenytoin, salicylate, theophylline, tobramycin, valproic acid, vancomycin, voriconazole). Three drugs displayed strong decreases in measured concentrations after storage in one or both gel-containing tubes (total tricyclics, lidocaine, and free phenytoin). Following adoption of gel-containing tubes, 94% of the five most frequently ordered drug monitoring tests in the Emergency Department were collected in serum separator tubes. Evaluation of the stability and accuracy of commonly monitored drugs revealed that the majority were not affected by exposure to gel separator material under conditions similar to outpatient clinic storage, courier transport, and laboratory storage. Expanding the collection requirements for appropriate drugs to include gel separator tubes decreases the number of specimens drawn and the complexity of laboratory workflows.