Spuriously low plasma propranolol concentrations resulting from blood collection methods.

Spuriously low values of plasma propranolol concentrations resulted when one commercial brand of collection tube was used. The mechanism of this effect was established and was due to the presence of the stopper containing substances that reduced plasma propranolol binding and resulted in a redistribution of the drug in whole blood so that more was present in the cells and less in the plasma. A similar phenomenon was noted some years ago when a needle for intravenous infusion was used as an indwelling cannula. Th mechanism in this case could not be established as in the interim the phenomenon was no longer detectable. Although it was established that these findings do not apply to all drugs, the data suggest that collection methods should be routinely checked at intervals unless separate venipuncture and all glass collection tubes are used.