Hepatitis C tests, blood

• Emergency HCV antibody screen: Unlike the HIV situation, there is no such test for HCV.
• Elective HCV antibody screen: Use local EIA test for HCV antibodies (to a single recombinant protein reagent antigen...c100-3) to screen those with high risk factors (see below) and those with abnormal LFTs (if any risk factors).
  1. LML's  EIA 2nd generation, multiple-antigen test is best diagnostic screen test because less false positives
  2. 3rd gen. more sensitive but less specific (more false positives) and more ideal for screening donor blood units
• HCV antibody supplemental test: tests by RIBA (recombinant immunoblot assay, originally a supplemental...not confirmatory...test) apply patient's antibody-containing serum to bands of 4 HCV proteins in a strip...might use it when a surprise EIA screen positive in a low risk patient. "Positive" is reactivity to 2 or more antigens & some feel this 2nd gen. (RIBA-2 SIA) test is confirmatory when reactive to >2 antigens.
• HCV confirmatory test:  
  • prior to the RNA tests, the Western Blot test (denatured HCV is antigen reagent)...detecting circulating HCV antibody in our most specific (least false positive rate) fashion at the time...was the confirmatory lab diagnostic test.
  • HCV RNA detection (quant): If EIA test positive, get HCV RNA by PCR or TMA or b-DNA quantitative (sensitive to as low as 550-600 IU/mL)...if it is negative,
  • HCV RNA detection (qual): If EIA test negative, get HCV RNA by TMA or PCR qualitative (sensitive to as low as 10-100 IU/mL)...if it is negative,
  • see RIBA, above.
• "Indeterminate" test results: this problem has to be addressed with retesting or serial testing, alternate testing, clinicopathological correlation, etc.
• Reference Labs: we use LML for EIA and Specialty Lab for TMA, PCR, and b-DNA, and info is on their site...put HCV into their search engine; a type of testing flow-chart is located at another lab site, Quest Diagnostics
• Discordant test results: Is this an HCV-positive patient or not!? One can end up with a mix of positive and negative results depending on whether you are comparing tests for antibodies vs.  tests for circulating HCV RNA. And, there can be differences in sensitivity and false positive rates between types of antibody tests, types of HCV protein tests and types of HCV RNA tests. Keep in mind that: (1) all tests have false positives, (b) some HCV-infected patients cannot (or have not yet) mounted a detectible antibody response, and (c) some HCV-infected patients flux in and out of periods of viremia (RNA detectibility). Up to 15% of chronically HCV-infected individuals followed over time will have periods of viral RNA antigen "drop out" (negativity for RNA detectibility).

References:

1. CAP TODAY 16(4), April 2002.
2. Urdea MS, [of Chiron Corp.] "Hepatitis C-diagnosis and monitoring", Clinical Chemistry 43(8):1507-1511, 1997.

(posted May 2002; latest update, 15 October 2002)