The concentration of a medication in the blood that will inhibit the replication of a specified percentage of microorganisms. The abbreviation IC is often followed by a number, e.g., 50 or 90. The IC50 of a drug to treat acquired immunodeficiency syndrome is the drug concentration that will inhibit replication of 50% of all HIV virions, and IC90 is the concentration that will inhibit 90% of the virions.