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What Is Viral Load?


Updated May 19, 2014

Viral load is a measurement of the amount of virus in your blood. Viral load measurements are commonly used to monitor chronic viral diseases, such as HIV, hepatitis B (HBV) and hepatitis C (HCV).

In the case of HCV, a test called quantitative HCV RNA is used to measure the number of hepatitis viruses in the blood based on how much viral RNA (the virus' genetic material) is detected per milliliter. There is also a viral load test for hepatitis B.

Why Is Viral Load Important?

Doctors use your viral load to determine how you're responding to treatment. Usually, you will find out your viral load when you start treatment and then repeat the test (usually at three months) as your treatment continues. With at least two viral load results (sometimes along with the results of other blood tests and biopsy), doctors can see how the virus is responding to the drugs. A significantly reduced viral load (a 100-fold decrease in amount of virus) often means that treatment is working. Ideally, treatment should make the virus undetectable.

A viral load test won't tell you about the severity of your liver disease. In other words, a high viral load doesn't necessarily mean you're more sick than someone with a lower viral load. However, viral load can predict how easily you might achieve an undetectable viral level with treatment. Someone who begins treatment with a low viral load will probably have an easier time of getting an undetectable viral level after treatment.

Treating Hepatitis C

What Is a High and Low Viral Load?

A high viral load is usually higher than 800,000 IU/L. This means you have around 2 million copies or more of virus in your blood. A low viral load is usually lower than 800,000 IU/L. However, this is a general guideline and may be defined differently in certain people.

Laboratories standardize viral load results into international units (IUs), which allows you to compare a viral load result from one laboratory to that from another. A viral load of 800,000 IU/L doesn't mean you have 800,000 copies of virus; rather, you have 800,000 standardized units of virus per liter of blood. The actual number of virus copies may be higher or lower. For example a viral load of 50 to 100 copies per mililiter would be equivalent to 25 to 50 IU/mL.

What Does It Mean to Have an Undetectable Level?

An undetectable level means that the viral load test isn't able to detect any virus. However, you can have an undetectable viral load but still test positive for viral hepatitis. This is because viral load tests have a limit to how few viruses they can detect. Any amount of viruses fewer than that limit will be undetectable. Tests vary in terms of sensitivity. For instance, while some viral load tests can detect as little as 50 IU of virus per mL, others can only go as low as 615 IU/mL.

For hepatitis C patients, an undetectable viral load six months after treatment is called sustained virologic response (SVR). This means that the virus is in remission, which is as close to a cure as you can achieve with current treatment options.

How Are Viruses Detected?

Viruses are detected using a laboratory process called polymerase chain reaction (PCR). With this process, a very small amount of the virus' genetic material can be detected and measured. There are additional techniques that can also detect viral genetic material, including transcription mediated amplification (TMA) and branched chained DNA (bDNA), but PCR is the most common method.


Lab Tests Online. Hepatitis C Tests. Accessed on September 13, 2009.

National Digestive Diseases Information Clearinghouse. Chronic Hepatitis C: Current Disease Management.

United States Department of Veterans Affairs. Understanding Lab Tests. Accessed on September 13, 2009.

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