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HCV Viral load tests

I made a viral load calculator so you can check for yourself.

Viral load tests are blood tests that measure HCV ribonucleic acid (RNA, or genetic material) in the blood. The presence of viral RNA indicates that the virus is actively replicating (reproducing and infecting new cells). A viral load test is usually first done after a person has tested positive for exposure to HCV based on an antibody test. A blood sample is taken and the amount of HCV RNA in a milliliter of blood is measured. Viral load tests confirm whether an individual is actively infected with HCV. Viral load test results were previously measured in number of copies, but are now typically reported in terms of International Units per milliliter (IU/mL).

Types of HCV Viral Load Tests
There are two categories of HCV viral load tests:

Qualitative viral load tests — These tests determine the presence of HCV RNA in the blood. This type of test is usually used to confirm chronic infection with HCV. If viral RNA is detected, a positive result is reported; if viral RNA is not detected, the test result is negative.

Quantitative viral load tests — These tests measure the amount of virus in one milliliter of blood. They are often used to assess whether or not treatment with interferon or interferon plus ribavirin is likely to be successful and, later, if treatment is working.

There are currently three tests commonly used for HCV viral load testing:

Polymerase chain reaction (PCR) — PCR tests detect HCV RNA in the blood, which indicates current active infection. This type of quantitative PCR test is very sensitive, and can measure as few as 50 IU/mL.

Branched-chain DNA (bDNA) — The bDNA method quantitative viral load testing is easier (and cheaper) to use for a large number of samples, but only measures viral loads greater than 500 IU/mL. This means that if a person has a viral load below 500 IU/mL, HCV could be present in the blood but not detected by the test.

Transcription-mediated amplification (TMA) — TMA technology allows for the amplification and detection of nucleic acids (components of genetic material) in the blood. This test can measure as few as 5-10 IU/mL. This newer test appears easier and cheaper to use, streamlining test processing and producing consistent, reliable, and more rapid results.

Interpreting Viral Load Test Results
HCV viral load is often reported as low or high.
Expressed as copies/mL:

  • Low: less than 2 million copies
  • High: more than 2 million copies

Expressed as International Units (IU/mL):

  • Low:less than 800,000 IU/mL
  • High:more than 800,000 IU/mL

If no HCV RNA is found by a test, a person’s viral load is said to be undetectable. Note that whether viral load is undetectable depends on which test is used. PCR and TMA tests can measure viral loads much lower than those a bDNA test can detect. Importantly, the blood of an individual with a very low viral load may still contain HCV even though the current tests cannot measure it; that is, the virus may not have been truly eradicated from the body.

Viral load test results can vary depending on how a blood sample is handled and stored. Furthermore, results may vary from lab to lab. For this reason, most experts recommend that people should get their viral load testing done by the same laboratory each time, so that results are more comparable.

Changes in viral load are sometimes expressed in terms of logs. A log change is a 10-fold increase or decrease. For example, a change from 1,000,000 IU/mL to 10,000 IU/mL is a 2-log decrease.

Converting copies per milliliter to Inter-national Units
There is no standard conversion formula for converting the amount of HCV RNA reported in copies per milliliter to the amount reported in International Units. The conversion factor ranges from about one to about five HCV RNA copies per IU. Usually the lab report will list the conversion from IU/mL to copies/mL. See Table 1 for a conversion of common viral load tests from IUs to copies.

Table 1: Conversion Chart

Screen Shot 2013-06-10 at 4.07.05 PMUses of Viral Load Test Results

Viral load test results have many uses, such as confirming active HCV infection, and predicting and measuring HCV treatment response before, during, and after therapy. Higher HCV viral loads may be associated with a greater risk of HCV transmission, particularly transmission from mothers to infants during pregnancy or birth. Viral load has not been correlated with the risk of sexual transmission. Furthermore, a correlation between HCV viral load and disease progression has not been shown.

Confirming active HCV infection — After a person has tested positive for HCV antibodies, an HCV viral load test is usually performed to confirm active HCV infection. This test is necessary because in up to 25% of people exposed to HCV, the virus can be cleared on its own.

Before treatment — Viral load measurement can help predict how well HCV treatment will work.

The lower the pre-treatment viral load, the more likely it is that a person will respond to current HCV therapies.

During treatment — A decrease in viral load while on therapy indicates that treatment is working. A treatment is said to produce a complete virological response if it reduces viral load to an undetectable level. After 12 weeks of antiviral treatment, a 2-log drop in viral load or elimination of detectable HCV is an indication that the medications are working. If a person does not achieve a 2-log drop in viral load or elimination of detectable HCV after 12 weeks, it is unlikely that he or she will be able to eradicate HCV from his or her body. Viral load tests during treatment can also detect viral breakthrough, or increases in viral load that occur after a previous undetectable test result.

Note: A log drop in viral load is measured by decreasing the number by one zero. For instance, a one log drop in a viral load of 1,000,000 International Units is 100,000 International Units; a two log drop in a viral load of 1,000,000 International Units is 10,000 International Units.

After treatment — Viral load measurements can be used after cessation of therapy to monitor for relapse—that is, to see if the virus becomes detectable again after being undetectable when treatment was completed.

 

2 replies
  1. girlsmadrid
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