What Is a Viral Load?

Measuring Viral Activity Tells Us How Effectively a Treatment Is Working

Electron micrograph of hepatitis C virion (HCV). Photo Credit: BSIP/UIG/Getty Images

A viral load is simply the 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 a quantitative HCV RNA assay is used to measure the amount of virus' genetic material (RNA) detected in a milliliter of blood. Other technologies can be also used to monitor viral activity, most of which do so by detecting either viral DNA or RNA.

Why Is a Viral Load Important?

Doctors commonly use your viral load to determine how well you're responding to treatment. Typically your viral load will be tested before you start therapy (for hepatitis C, for instance) and then repeated periodically to measure how effectively you are responding to treatment. At least two viral load results are needed to assess treatment efficacy.

A significantly reduced viral load (i.e., a 100-fold decrease in viral actively) generally means that treatment is working. Ideally, a person would achieve a so-called "undetectable" viral load, meaning that the current testing technologies are unable to find any evidence of virus in blood samples.

Every ten-fold increase or decrease in viral activity—from, say, 100 or 1,000 or, conversely, 1,000 to 100—is considered a one-log change in viral load.

While the test in valuable in predicting treatment outcomes, it does not tell you anything about the severity of your liver disease.

Typically, biopsies and other testing technologies (e.g., ultrasound, MRI) are needed for that.

High vs. Low Viral Load

Insofar as viral hepatitis is concerned, a high viral load is usually that over 800,000 IU/L, while a low viral load is under 800,000 IU/L. This range can vary significantly, however, based on what is considered average in a specific region or population.

It's important to note, however, that a viral load of 800,000 IU/L does not mean that you have 800,000 actual viruses in your blood; rather, it means that lab has determined that there are 800,000 international units (IU) in a liter of blood. An IU is a standard measurement used by pathologists to ensure consistency from lab to lab and are consider more accurate than a simple "head count" of viral RNA.

Typically speaking, though, 800,000 IU/L correlates to around two million copies of viral RNA. 

What Undetectable Viral Loads Tell Us

An undetectable viral load does necessarily mean you have no virus in your blood or that you've achieved a cure. However, if following treatment, you are able to sustain an undetectable viral load—typically for a period of 24 weeks—the likelihood of the virus reappearing (rebounding) is consider extremely low.  Low enough, in fact, for a person to be technically considered cured.

This period of remission following hepatitis therapy is known as a sustained virologic response (SVR).

The gold standard for treatment success is classified as SVR-24, meaning that has been no viral activity detected after 24 weeks.

While tests can vary in terms of sensitivity (i.e., their ability to detect virus), most current assays are extremely accurate.

How Are Viral Loads Measured?

Viral loads are performed using a laboratory technology known as polymerase chain reaction (PCR). With this process, a very small amount of the virus' genetic material is copied repeatedly (amplified) until there is enough to make an accurate analysis. 

While PCR is the most common test used for measuring hepatitis virus, there are other techniques that also detect the viral genetic material, including transcription mediated amplification (TMA) and branched chained DNA (bDNA).


U.S. Department of Veterans Affairs. "Understanding Lab Tests." Washington, D.C.; accessed September 13, 2009.

Gupta, E.; Bajpai, M.; and Choudhary, A. "Hepatitis C virus: Screening, diagnosis, and interpretation of laboratory assays." Asian J Transfus Sci. Jan-July 2014; 8(1).

Welzel, T.; Miley, W.; Parks, T.; et al. "Real-Time PCR Assay for Detection and Quantification of Hepatitis B Virus Genotypes A to G." J. Clin Microbiol. September 2006; 44(9): 3325-33.

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