At least six major genotypes of Hepatitis C (HCV), each with multiple subtypes, have been identified worldwide. Genotypes are designated with numbers (genotypes 1-6) and subtypes with letters. HCV genotypes 1, 2, and 3 appear to have a worldwide distribution, but their prevalence varies from one geographic area to another.
- Subtypes 1a and 1b are the most common genotypes in the United States and Europe.
- Subtype 1b is the most frequent subtype in Japan.
- Subtypes 2a and 2b are found in North America, Europe, and Japan.
- Subtype 2c is prevalent in northern Italy.
- Subtype 3a is detected in IV drug users in Europe and the United States.
- Genotype 4 is identified in North Africa and the Middle East.
- Genotypes 5 and 6 are predominant in South Africa and Hong Kong, respectively.
The United States has approximately:
- 70% genotype 1
- 16% genotype 2
- 13% genotype 3
- < 1% genotypes 4-6
HCV genotyping helps predict the outcome of therapy and influences the choice of treatment. Currently, the only clinically relevant distinction is between genotype 1 and genotypes 2 and 3. Studies have shown that patients infected with genotype 1 are less likely to have a sustained favorable response to treatment than patients with genotypes 2 and 3. Other predictors of response to antiviral therapy are viral load of less than 2,000,000 IU/mL, shorter duration of infection, female gender, and low body weight.
- With Pegylated IF/Ribivarin, a 2 log drop in viral load in first 12 wks indicates a sustained virologic response
- If virus is not undetectable by 24 weeks, a patient will most likely relapse
- Two patterns of no response are seen:
- Null response = don't see 2 log drop in first 12 weeks; stop therapy
- Partial response = achieve 2 log drop in first 12 weeks, but viral load stabilizes & doesn't decrease further; stop Rx at 24 wks because patients won't become viral negative
- Genotype 1 has 20% null responders
- 80% early response within 12 weeks
- 65% become viral negative
- 40 - 50% have a sustained viral response
- Genotype 2 has 3% null responders
- 97% have early virologic response
- 90% have sustained viral response
A single specimen can be used to determine both genotype and viral load. The specimen requirement is one SST, EDTA or ACD tube of blood. Samples collected in heparin tubes are unsuitable for analysis.
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