Effect of monitoring drug concentrations through lines used to administer the drugs: an in vitro study.

Elevated drug concentration (DC) can be caused by intentional or iatrogenic overdoses and sampling technique errors. This study examined, in vitro, technique factors that can cause and prevent false elevations of DCs when drugs are administered through the catheter lines from which blood samples are later taken for DC analysis. Digoxin, aminophylline, and phenytoin were administered through central catheters in concentrations simulating those used clinically. Drug solution remained in the lumen for a time similar to that encountered clinically, then either remained in the tubing or was flushed out with 5 ml of normal saline (NS). After 6 h, a 5-ml sample was withdrawn for DC analysis (the tip of the catheter placed in NS, which represented blood supply). Prior to final sample withdrawal, 5 ml of NS was drawn through half of the lines and discarded to simulate methods used to prevent contamination. Thus, various flushing/no flushing and sample discarding/no discarding techniques were analyzed. When diluted concentrations of drug were administered, minor artifactual DCs were observed unless the line was not flushed or sample fluid discarded prior to final sampling. With undiluted drug administration, only flushing after the dose and discarding a sample prior to final sampling prevented artifactual DCs. Clinicians should be aware of the potential for such artifactual elevations of DCs and should encourage techniques that prevent their occurrence.