To determine the potential for a drug or chemical to partition into red blood cells (RBC partitioning) a series of drug concentrations (e.g. 20, 10, 5, 2.5 µg/mL) are prepared by making dilutions into 6 mL of whole blood. The packed cell volume (PVC) of the test blood is determined. Once the blood-drug dilutions are made the samples are incubated for 24 hours at 37°C. After incubation, plasma and RBCs are separated by centrifugation of whole blood samples at 1200 rpm for 10 minutes. Aliquots of whole blood (after incubation), plasma, RBC pack and standard dilutions are analyzed by LC/MS/MS to determine the amount of drug present in each fraction. The concentration of drug is estimated based on calibration curves. Final results represent the average from triplicate experiments (Deshmukh et al., 2009).