In humans, ingested drugs are first absorbed through the gut and then enter the bloodstream, where they interact with blood plasma proteins. The extent of the plasma protein binding influences the drugs distribution and availability. For example, a compound that binds with high affinity to blood plasma proteins means there will be less free compound available for metabolism. This can reduce the therapeutic effects at a particular dose, and can affect clearance of the compound. IonTox uses solid phase microextraction (SPM) to assess plasma protein binding. The test compound is incubated with human (or rat) blood plasma and a buffered solution (PBS). After incubation, the presence of the test compound is analyzed by LC-MS/MS.