如何檢測C型肝炎病毒？ZT （英文）

[How is it used?]
Each of the five most common tests has a slightly different purpose:

* Anti-HCV tests detect the presence of antibodies to the virus, indicating exposure to HCV. These tests cannot tell if you still have an active viral infection, only that you were exposed to the virus in the past. Usually, the test is reported as “positive” or “negative.” There is some evidence that, if your test is “weakly positive,” it may not mean that you have been exposed to the HCV virus. The Centers for Disease Control and Prevention (CDC) revised its guidelines in 2003 and suggests that weakly positive tests be confirmed with the next test before being reported.

* HCV RIBA test is an additional test to confirm the presence of antibodies to the virus. In most cases, it can tell if the positive anti-HCV test was due to exposure to HCV (positive RIBA) or represents a false signal (negative RIBA). In a few cases, the results cannot answer this question (indeterminate RIBA). Like the anti-HCV test, the RIBA test cannot tell if you are currently infected, only that you have been exposed to the virus.

* HCV-RNA test identifies whether the virus is in your blood, indicating that you have an active infection with HCV. In the past, it was usually performed by a test called a qualitative HCV. Qualitative HCV RNA is reported as a “positive” or “detected” if any HCV viral RNA is found; otherwise, the report will be “negative” or “not detected”. The test may also be used after treatment to see if the virus has been eliminated from the body.

* Viral Load or Quantitative HCV tests measure the number of viral RNA particles in your blood. Viral load tests are often used before and during treatment to help determine response to treatment by comparing the amount of virus before and after treatment (usually after 3 months); successful treatment causes a decrease of 99% or more (2 logs) in viral load soon after starting treatment (as early as 4-12 weeks), and usually leads to viral load being not detected. Some newer viral load tests can detect very low amounts of viral RNA, and some laboratories no longer do qualitative HCV RNA tests if they use one of these versions of viral load testing.

* Viral genotyping is used to determine the kind, or genotype, of the virus present. There are 6 major types of HCV; the most common (genotype 1) is less likely to respond to treatment than genotypes 2 or 3 and usually requires longer therapy (48 weeks, versus 24 weeks for genotype 2 or 3). Genotyping is often ordered before treatment is started to give an idea of the likelihood of success and how long treatment may be needed.


[When is it ordered?]
Hepatitis C infection is the most common cause of chronic liver disease in North America; about 2% of all adults in the United States have been exposed to the virus, and 75-85% of those are chronically infected. The CDC recommends HCV testing in the following cases:

* If you have ever injected illegal drugs
* If you received a blood transfusion or organ transplant before July 1992*
* If you have received clotting factor concentrates produced before 1987
* If you were ever on long-term dialysis
* For children born to HCV-positive women
* For health care, emergency medicine, and public safety workers after needlesticks, sharps, or mucosal exposure to HCV-positive blood
* For people with evidence of chronic liver disease
* The blood supply has been monitored in the U.S. since 1990, and any units of blood that test positive for HCV are rejected for use in another person. The current risk of HCV infection from transfused blood is about 1 case per two million transfused units.

A positive anti-HCV test may be confirmed with an HCV RIBA test, especially if the test is “weakly positive.” Qualitative HCV-RNA is often used when the antibody test is positive to see if the infection is still present. HCV viral load and genotyping may be done to plan treatment; viral load and qualitative HCV RNA are also used to monitor response to treatment.

[What does the test result mean?]
If the antibody test result is positive, you have probably been infected with hepatitis C, even if it was so mild you did not realize you had it.

A positive RIBA confirms that you had been exposed to the virus, while a negative RIBA indicates that your first test was probably a false positive and you have never been infected by HCV.

A positive (or detectable) HCV RNA means that you are currently infected by HCV.