In addition to antiviral cytokines, test samples may also contain unknown viruses that potentially interfere with the bioassay. Treatment with heat or acid may be used to inactivate these viruses without affecting the bioactivity of type I and type III IFNs. However, as heat-inactivated influenza viruses still may be able to induce type I IFN [36] , virus inactivation by acid may be more convenient.