A method for evaluating the in vivo presence of a factor that prevents the biological effect of a type I (IFN) in an individual that is under treatment with type I interferon is described. The in vivo presence of antibodies directed against a type I interferon (IFN) is evaluated in an individual that is under treatment with type I interferon. The method includes incubating a blood sample of the individual in vitro with a suitable amount of the type I interferon for a suitable period of time, and determining mRNA levels of a biological marker of IFN activity, preferably MxA, in the blood sample. The treatment may involve a treatment of multiple sclerosis, HCV or HBV using a type I interferon.