One indicator of the expected clinical antiviral activity is the inhibitory quotient (IQ), defined herein as the ratio between the average total plasma concentration (C ave ) and the 50% effective inhibitory concentration (EC 50 ). In order to predict the effectiveness of the TCAD regimen against the circulating influenza strains in the clinical setting, we calculated and compared the IQs for amantadine, ribavirin, and oseltamivir carboxylate as single agents and the TCAD regimen against susceptible and resistant influenza viral strains (including 2009 H1N1). To determine the IQ of the TCAD regimen, amantadine, ribavirin, and oseltamivir carboxylate was tested as a fixed ratio combination, wherein the ratio of the three drugs was kept constant even as the total concentration of drugs varied. A dilution curve of the TCAD regimen was created by first preparing a solution of all three drugs at 100-fold the C ave of each drug (43 mg/mL amantadine, 130 mg/mL ribavirin, 30 mg/mL oseltamivir carboxylate), and then serially diluting this solution in half-log 10 increments. In this manner, the EC 50 of the TCAD regimen was determined as a ratio of the C ave and expressed in units of fold change from C ave .