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From the AASLD Conference, November 1995
HOW IS TESTING FOR HCV DONE AND WHAT IS THE SIGNIFICANCE
OF A POSITIVE TEST?
Screening tests for HCV are commonly done by "ELISA" methods in
which a bead containing recombinant HCV antigens are mixed with
serum from the person being tested. If the test serum contains
antibody directed against HCV, the antibody binds to the antigens
coating the bead. The beads are then washed to remove any unbound
materials and a detection antibody and a color producing substance
is added. If antibody is bound to the bead, we can measure the
amount of color and determine if the patient's serum is
infected.
Most ELISA tests in current use are very sensitive at picking up
antibodies to the hepatitis C virus, but they also have a high rate
of false positivity (cases where the test appears positive but the
patient is NOT really infected). To be certain that a positive test
is truly positive, most testing sites do further testing (called
RIBA or Recombinant ImmunoBlot Assay) on all positive sera. A test
is reported as positive only if it is confirmed by the RIBA assay.
In RIBA testing, recombinant HCV antigens are applied separately
onto special paper strips and used to detect the presence and
specificity of antibody present in the test serum. Typically, a
RIBA test is called positive, confirming HCV infection, if two or
more HCV antigens are detected. Third generation ELISA tests are
becoming available which are highly specific (99%) for HCV; they
are not yet FDA Licensed, Despite the high sensitivity of second
generation ELISA tests, antibody production by an infected
individual may take up to 3 to 4 months (rarely up to 6 months) to
develop. Hence, there remains a "window period" during which the
individual is infected but without detectable antibody.
Immunosuppressed patients (renal transplant, those on steroids, HIV
patients) may also have active HCV infection without detectable
antibody.
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