When patients who have never received antiviral therapy are given antiviral treatment, they showed three patterns of response to antiviral treatment. Each based on the effect of the treatment on the virus: sustained response, relapse, and non-response.

The optimal response to antiviral therapy is called a sustained response. A sustained response is defined as the absence of detectable hepatitis C virus RNA using the RT-PCR or TMA assay 6 months after treatment is stopped. Most of these individuals will remain in remission (no signs of the disease) indefinitely, with no detectable hepatitis C virus RNA in the blood or liver. Moreover, follow-up biopsies will show a marked reduction in inflammation and regression of scarring (fibrosis). A longer follow-up of these patients is necessary, however, to evaluate whether sustained responders will avoid the complications of cirrhosis and live longer.

Relapsers are patients who seem to respond initially to treatment, that is, whose hepatitis C virus RNA becomes undetectable during therapy, but then becomes detectable shortly after discontinuing therapy. The virus becomes detectable again within six months and usually within the first three months of stopping treatment. This type of response to antiviral treatment is known as a relapse.

Finally, patients who have detectable hepatitis C virus RNA during therapy are known as non-responders. This type of response to antiviral treatment actually is called a non-response. In addition, other patients in whom the hepatitis C virus RNA becomes undetectable during the early period of treatment but reappears before the end of therapy, should probably likewise be considered non-responders. This reappearance of hepatitis C virus RNA during therapy is referred to as a break through because the hepatitis C virus RNA breaks through what initially appears to be successful (loss of hepatitis C virus RNA) treatment.