Hepatitis C (cont.)

Several diagnostic tests currently are available for the diagnosis of HCV infection. They can be categorized according to the way the tests are used.

Screening tests are tests that are used to diagnose a condition or disease among individuals not known to have the disease. They are is particularly useful for individuals who have risk factors for the condition or disease.

The first step in screening for HCV infection is to test blood for the antibody to HCV using an enzyme immuno-assays (EIAs). If the EIA test is negative (does not find the antibody), the patient is assumed to be free of HCV. It takes several weeks (up to six months) for antibodies to develop after the initial infection with HCV, so this screening test may miss a few newly-infected individuals. The EIA screening tests are very good (specific); if the test is positive the probability of having HCV infection is greater than 99%.

Recombinant immunoblot assay (RIBA) is used to confirm the positive results of EIAs since occasionally a positive EIA is a false positive, that is, the test is positive when HCV is not present. Although the direct detection of HCV RNA (HCV PCR) also is widely used to confirm the HCV infection, RIBA is still useful to differentiate false positive results in the few individuals whose immune systems have eliminated the virus but still have antibodies left over from the resolved infection.

As previously described, HCV contains RNA. Several tests (assays) are available to measure the amount of HCV RNA in a person's blood. These tests are referred to as molecular tests because they examine the virus at the molecular level. A single negative test for RNA does not mean that there is no infection because the virus may appear in the blood intermittently or may exist in small amounts. Newer tests have helped by detecting smaller and smaller amounts of virus in the blood.

Testing for RNA is useful in determining whether or not a patient has circulating virus in the blood (viremia). Hence, it can be used to confirm that a positive EIA/ELISA truly reflects active HCV infection.

RNA testing also should be done in individuals who may have been recently exposed to HCV. HCV RNA testing is more sensitive (that is, will detect more cases) than the conventional EIA testing in this setting. The reason for this greater sensitivity is that it may take a person several weeks after exposure to HCV to develop the antibodies, whereas HCV RNA becomes detectable one to three weeks after exposure. Finally, HCV RNA testing may be helpful to assess a patient's response to treatment at certain time points during antiviral therapy (see treatment of HCV below).

Blood tests also have been developed to identify the HCV genotype. This information is used to help guide treatment.

The table below provides guidelines for interpreting the results of testing for HCV antibodies by EIA and RIBA and for hepatitis C virus RNA. These are standard interpretations, but it is important to remember that the diagnosis of HCV infection should be made by an experienced clinician who is familiar with the patient's medical history.

Source: MedicineNet.com
http://www.medicinenet.com/hepatitis_c/article.htm