What Is a Viral Load And Why Is It Important?

Measurement of Viral Actvity Is the Key Indicator of Treatment Success

Blood Test used under Creative Commons license https://www.flickr.com/photos/lori_greig/4185759295
Photograph © Lori Greig

Question: What is a viral load, and why is it important?

Answer: The HIV viral load is an important measurement of the amount of active HIV circulating in the blood of someone who is HIV-positive. The viral load is used to determine how effectively your antiretroviral therapy (ART) is working, including whether you are developing resistance to your HIV medications or are not fully suppressing the virus due to suboptimal drug adherence.

Additionally, the suppression of viral activity directly correlates to a reduction in the infectivity of HIV-positive individuals, a preventive strategy popularly known as Treatment as Prevention (TasP).

The Goal of HIV Therapy

The goal of HIV therapy is to suppress replication of the virus in order to bring the viral load down to undetectable level.

Undetectable does not mean no virus or that the virus has been cleared from your body. It simply means that no virus can be detected using currently available testing technologies. Once ART is stopped the virus will rebound, meaning that it is once again replicating and growing in population.

It is important to note that viral loads can vary from blood (serum) samples and other body fluids. For example, the viral load might be undetectable in blood but remain detectable semen, the latter of which may increase the potential for transmission to an uninfected person.

People with higher viral loads also have a greater risk for immune system damage, which in turns leaves the body at risk for the development of opportunistic infections.

How Viral Load Testing Is Done

Typically, your doctor will draw blood about once every three to six months to measure the amount of viral activity.

The test results can vary depending on the viral load test being used.

While less commonly used in the developed world, quantitative HIV viral load tests (used to measure the quantity of virus) measures viral from over 750,000 copies/mL to less than 400 copies/mL. Newer ultra-sensitive viral load assays can measure viral activity well over 1,000,000 copies/mL to as little as 5 copies/mL.

Meanwhile, qualitative HIV viral load tests (used to qualify infection) is simply used to confirm that an HIV infection has taken place and is particularly important in testing infants and newborns born to HIV-positive mothers.

Interpreting Viral Load Results

Simply put, the fewer copies of HIV in your blood the better.

When starting treatment, viral load tests provide the baseline measure which is then compared to tests performed after ART is started. Every ten-fold drop in viral load is considered a one log drop, For example, if the viral load drops from 50,000 copies/mL to 500 copies/mL, the patient is said to have a two log drop in viral load.

Generally speaking, with current generation antiretroviral drugs, one can expect to achieve an undetectable viral load in anywhere from 3-9 months.

However, if the patient has or develops resistance to any of the HIV drugs, completely viral suppression may not able be achieved. In such case, treatment may need to be change, directed by a genetic resistance test used to determine which drug or drugs the patient is resistant to.

The aim, of course, is to sustain undetectable viral loads for many years, which not only preserves your treatment options over the long term but directly confers to a 53% reduction of HIV- and non-HIV-associated illnesses.

Furthermore, a sustained undetectable viral load can achieve the goals of TasP by lowering the risk of transmission between mixed-status (serodiscordant) couples by as much as 96%.


Cohen, M.; Chen, Y.; McCauley, M.; et al. "Prevention of HIV-1 infection with early antiretroviral therapy." New England Journal of Medicine. August 11, 2011; 365(6):493-505.

National Institutes of Health (NIH). "Starting antiretroviral therapy early improves outcomes for HIV-infected individuals." Bethesda, Maryland; issued May 27, 2015.

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