Methods for isothermal titration calorimetry analysis of the binding affinity of protease inhibitors to plasma proteins. A method that can quantitatively calculate free drug concentrations of protease inhibitors in human plasma, as well as a method to calculate therapeutic amounts and dosage regimens. Furthermore, the present invention provides a method that can calculate the effect of plasma proteins on the antiviral activity (EC50 values) of protease inhibitors from their binding affinities to plasma proteins. The present invention provides as well a method that can evaluate the in vivo anti-HIV efficacy of PIs in human plasma.