Emulsion effect on the measurement of standard laboratory blood parameters

Background: A number of pharmaceutical agents have limited water solubility and are therefore often prepared in a lipid emulsion. Emulsion renders plasma opaque and this could interfere with the accuracy of some standardized laboratory measurements, especially for optical or mechanical based assays. We determined the interference on some laboratory diagnostic values of blood specimens after propofol addition in vitro as well as in vivo when infused into swine. Methods: In vitro, laboratory parameters were measured immediately after mixing swine blood diluted with increasing amounts of propofol emulsion in the range of 3 to 23%, v/v. The contact time of a 9% v/v mixture of blood and propofol was also examined over a 3-hours period. Saline-diluted samples served as controls. Cellular volume, hematocrit, hemoglobin, potassium, and coagulation were measured with various instruments. In addition, similar parameters were analyzed from swine blood following a 9 - 10 hours infusion with propofol/fentanyl compared to infusion with ketamine/midazolam. Results: In vitro, blood cell volume increased immediately upon contact with a mixture exceeding 6% propofol. Above 9% of the mixture, the cellular volume expanded significantly with extracellular K⁺ leakage. Hematocrit increased but the hemoglobin was dependent on the instrument type. Coagulation was altered when the emulsion was present. Interestingly, in vivo these effects were significant less pronounced in blood collected from experimental swine under total intravenous propofol anesthesia, but they departed from water-based anesthesia in control swine. Conclusions: The in vitro studies indicate that results from certain assays of blood samples rich in lipid emulsion may not be accurate due to interference with optical, mechanical or ion selective electrode methodology. Although in vivo samples may be less impacted, there is still a risk of deviation from accuracy.