A 10 µl sample of 0.5µM Epirubicin (EPI) in methanol was spiked into 100µl clinical plasma samples to provide an internal standard. EPI was chosen because it is an isomeric form of DOX that has very similar chemical and fluorescence properties to both DOX and DOL. The difference between the recovered amount of EPI and the known amount of EPI spiked into the plasma sample helped to scale the observed DOX and DOL concentration values to account for variability in the extraction efficiency as well as effects on overall fluorescence levels that could potentially be influenced by the buffering properties of the plasma.