Viral load testing
Viral load testing is a blood test used to determine the presence and measure the level of hepatitis C (HCV) in the blood. If you tested positive for HCV antibodies, you know that you were exposed to HCV. In a certain percentage of people who are exposed to HCV, the body clears the virus on its own. However, antibodies to HCV are still detectable. It is necessary to do a viral load test, then, to determine if HCV is still in your body.
The way to determine whether HCV is in your body is to look for its RNA. HCV RNA is the genetic material that the virus uses to reproduce itself. One of several different lab tests are used to detect HCV RNA in the blood as well as to measure the amount of the virus in your body (called the viral load). In addition to helping diagnose HCV infection, a viral load test can be used to 1:
- Monitor response to treatment.
- Determine whether the virus has been eliminated following treatment (usually 12 to 24 weeks after treatment).
- Predict how well treatment will work. A low pre-treatment viral load increases the chances of cure. However, with newer HCV treatments, this information is less useful.
Handling and storage of the blood sample can affect the results of the test. Also, results can vary from lab to lab. This is why doctors prefer to use the same lab for testing, so that results are comparable over time.
What do viral load test results mean?
Viral load testing can be confusing. The results do not correlate to symptoms or the amount of damage HCV may be doing to your liver. A high viral load doesn’t mean that you are getting worse. If your viral load is lower than a previous test, and you are not on HCV treatment, it does not mean that you are getting better.
When you have a viral load test, the results are typically reported in international units per milliliter (IU/mL) and copies per milliliter. A viral load is considered:
- High if greater than 800,000 IU/mL
- Low if less than 800,000 IU/mL
If viral load testing does not find any HCV, your viral load is called “undetectable.”
Fluctuations in viral load, as can be seen during treatment, are sometimes expressed using a term called “log”. A “log” drop in viral load is a 10-fold decrease. For instance, if you start with a viral load of 1,000,000, a one-log drop will give you a viral load of 100,000 (take away 1 zero) and a two-log drop will give you a viral load of 10,000.
Different types of viral load testing
There are two approaches to measuring viral load: quantitative testing and qualitative testing. Quantitative testing uses different techniques to measure viral load, including polymerase chain reaction (PCR), branched-chain DNA (bDNA), and transcription-mediated amplification (TMA). Both PCR (real-time) and TMA measure HCV RNA levels in the blood and are accurate down to 10-15 IU/mL. The bDNA is less complex than PCR and TMI, and is limited to measurement of viral loads above 615 IU/mL. 1
Quantitative testing is typically used before starting treatment to determine a baseline viral load for the purpose of evaluating treatment response. Qualitative testing is capable of detecting low levels of HCV RNA. It gives results as positive or negative and, in some cases, has a lower limit of detection below 10 IU/mL. Qualitative testing is typically used to confirm diagnosis, where a specific viral count is not needed. 1