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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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