Antibody Testing
The initial screening test for hepatitis C is a simple blood test. In response to the hepatitis C virus the body's immune system produces antibodies. The antibody test does not look for the virus itself but for the antibodies raised against the hepatitis C virus. Antibodies are specific to particular viruses. They usually will remain in the body for many years in an attempt to guard against further infection from that virus.
Due to the nature of the hepatitis C virus, the immune system is only able to eliminate the virus for around 25-30% of people. Hepatitis C antibodies will also not prevent someone from becoming infected with the hepatitis C virus again, or with a different genotype (strain) of the hepatitis C virus. The antibody test therefore indicates whether he person has at any time been infected with he virus. It does not determine whether someone still has the hepatitis C virus since the antibodies will remain in the blood even if the body has cleared the virus. The test also cannot tell how long ago the person was infected or for how long they might have had hepatitis C.
After being infected with the hepatitis C virus, it may take the body several months (usually around 2-3 months) to sero-convert, which means to develop antibodies which can be detected by the antibody test. This period before the antibody response fully develops is called the window period. During this time, someone with a recent infection could still show a negative or indeterminate antibody test, since his or her antibodies are still developing. Antibody test results are usually clearly positive or negative, but sometimes they come back as indeterminate. In this case, a follow-up test may be required. A repeat test is always performed to confirm positive results.
source - Impact booklet, produced by Hepatitis C Victoria, June 2008