Hepatitis C (cont.)
In this Article
Who is at high risk and should be tested for hepatitis C infection?
Currently, screening for HCV is not recommended as part
of a routine physical examination.
- Rather, testing should be done in individuals
at high risk for infection including current and past users of injectable drugs
and persons exposed to infected blood or organs from infected persons.
- Children
born to chronically infected mothers should be tested to determine if they carry
the virus.
- Persons with abnormal levels of liver enzymes in the blood also should be
tested.
These are not the only circumstances under which tests may be done. In
general, testing is recommended when exposure to the virus is suspected.
What are the diagnostic tests for hepatitis C virus and how are they used to diagnose hepatitis C virus infection?
Several diagnostic tests currently are available for the
diagnosis of HCV.
They can be categorized according to the way the tests are used.
Screening tests
Screening tests are tests that are used to diagnose a
condition or disease among individuals not known to have the disease. It is
particularly useful for individuals who have risk factors for the condition or
disease. The first step in screening for HCV is to test blood for the antibody
to HCV using an
enzyme-linked immunosorbent assay (ELISA). If the ELISA 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. However, in HCV physicians usually are looking for chronic
infection so missing a few recently-infected individuals is not of much
importance.
The ELISA test for HCV antibody is not perfect and may
sometimes be positive in people who are not currently infected. Thus, if the
ELISA test for HCV antibody is positive, additional testing is done to confirm
the diagnosis with another type of test for the antibody. One such test is
called the recombinant immunoblot assay (RIBA). It is read as either 'positive',
'negative' or
'indeterminate'. Indeterminate means that the result is unclear. Another option
is to use molecular tests to measure the amount of HCV ribonucleic acid (RNA) in
the blood (see below).
Molecular tests for hepatitis C virus
As previously described, HCV is an RNA virus, meaning that it 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 ELISA truly reflects active hepatitis C virus 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 ELISA 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 virologic response to treatment at certain time points during
antiviral therapy (see treatment of HCV below).
How are the results of the hepatitis C virus tests interpreted?
The table below provides guidelines for interpreting the results of testing
for anti-hepatitis C virus by ELISA and RIBA and for hepatitis C virus RNA.
These are standard interpretations, but it is important to remember that the
diagnosis of hepatitis C should be made by an experienced clinician who is
familiar with the patient's medical history.
| Anti-HCV (ELISA) |
Anti-HCV (RIBA) |
HCV RNA |
Interpretation |
| Negative |
Negative |
Negative |
No infection |
| Positive |
Positive |
Positive |
Ongoing infection |
| Positive |
Positive |
Negative |
Past or current infection. Additional or repeat testing should be done
to exclude fluctuating or low levels of viremia. |
| Positive |
Negative |
Positive |
False positive ELISA; no infection |
| Positive |
Indeterminate |
Negative |
Situation unclear, consider additional testing |
| Negative |
Negative |
Positive |
New (acute) HCV infection or chronic HCV infection in an
immunocompromised person unable to make adequate antibodies. |
What tests identify the virus genotypes?
Blood tests have been developed to identify the HCV genotype. This
information is used to help guide treatment.
Next: What is the role of a liver biopsy in the management of chronic hepatitis C? »
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