Measurement of propofol concentration in sheep blood and plasma: effect of storage at different temperatures

In planning a study of the pharmacokinetics of propofol in sheep, contradictions were noted in the literature with regard to loss of propofol during storage of blood samples. This prompted a study of such loss from samples of sheep blood and plasma during storage at room temperature, +4 degrees C and -20 degrees C, for up to 17 days, over a range of concentrations from 1 to 20 micrograms/mL. Samples were drawn from 22 different sheep. Analysis was by the method of Adam et al. (1981). The best estimate of the overall mean loss rate was 0.7% per day with 95% confidence limits of 0.3% to 1.2% per day. The loss rate increased nonsignificantly with storage temperature. There were very small nonsignificant differences of loss rate between plasma and blood, between different concentrations, and between genders. There were significant differences of loss rate between sheep--up to about 2% per day in blood or plasma from any one sheep.