Serologic tests are the most accurate method of identifying a viral cause for a liver disorder and distinguishing between the different forms of viral hepatitis. Although such characteristics of viral hepatitis as route of transmission and incubation period may be helpful in differential diagnosis, they seldom are sufficient to make an exact diagnosis. Laboratory tests or liver biopsy may help to differentiate liver disorders but again usually do not identify the form of viral hepatitis that is present. However, liver biopsy is often of value in patients with chronic hepatitis to determine the severity of disease, aid in prognosis, and, in some cases, serve as a guide to antiviral therapy.