CD4 vs. Viral Load: What’s in a Number?

Medically reviewed by Daniel Murrell, MD on February 16, 2018Written by Aaron Moncivaiz and Dorian Alexander on July 18, 2013

CD4 count and viral load

If someone has received an HIV diagnosis, there are two things they’ll want to know: their CD4 count and their viral load. These values give them and their healthcare provider important information about:

  • the health of their immune system
  • the progression of HIV in their body
  • how their body responds to HIV therapy
  • how the virus itself responds to HIV therapy

What is a CD4 count?

A CD4 count is a blood test to check the amount of CD4 cells in the body. CD4 cells are a type of white blood cell (WBC). They play a key role in the immune system. They alert other immune cells to the presence of infections such as bacteria and other viruses in the body. CD4 cells are also a subset of immune cells called T cells.

When a person is living with HIV, the virus attacks the CD4 cells in their blood. This process damages CD4 cells and causes the number of them in the body to drop, making it difficult to fight infections.

CD4 counts show the robustness of the immune system. A healthy immune system normally has a CD4 count ranging from 500 to 1,600 cells per cubic millimeter of blood (cells/mm3), according to HIV.gov.

When a CD4 count is lower than 200 cell/mm3, a person will receive a diagnosis of AIDS. AIDS occurs in stage 3 of HIV. At this stage, the body’s immune system is weak due to the low number of CD4 cells available to fight disease.

What is a viral load?

An HIV viral load test measures the number of HIV particles in a milliliter (mL) of blood. These particles are also known as “copies.” The test assesses the progression of HIV in the body. It’s also useful in seeing how well a person’s HIV therapy is controlling HIV in their body.

A high viral load may indicate a recent HIV transmission, or HIV that’s untreated or uncontrolled. Viral loads are generally highest for a period right after contracting HIV. They decrease as the body’s immune system fights against HIV, but then increase again over time as CD4 cells die off. A viral load can include millions of copies per mL of blood, especially when the virus is first contracted.

A low viral load indicates relatively few copies of HIV in the blood. If an HIV treatment plan is effective, a person will be able to maintain a lower viral load.

What’s the relationship between the two?

There’s no direct relationship between CD4 count and viral load. However, in general, a high CD4 count and a low — or undetectable — viral load are desirable. The higher the CD4 count, the healthier the immune system. The lower the viral load, the likelier it is that HIV therapy is working.

When HIV invades healthy CD4 cells, the virus turns them into factories to make new copies of HIV before destroying them. When HIV remains untreated, the CD4 count decreases and the viral load increases.

How often might someone be tested?

A healthcare provider will likely conduct CD4 counts and viral load tests more often at the beginning of HIV therapy or with any changes in medications. Most people living with HIV should have lab tests performed every three to four months, according to current lab test guidelines.

More frequent testing may be needed for some people, such as those in their first two years of treatment or those whose viral load isn’t suppressed. Less frequent testing may be needed for people who take daily medication or have maintained a suppressed viral load for over 2 years. They may only need to be tested twice a year.

Why is it important to get tested regularly?

A single CD4 or viral load test result only represents a snapshot in time. It’s important to track both of these and consider trends in test results rather than only looking at individual test results.

Keep in mind that these values may vary for many reasons, even throughout the day. The time of day, any illnesses, and recent vaccinations can all affect CD4 count and viral load. Unless the CD4 count is very low, this fluctuation isn’t usually worrisome.

Regular viral load tests, not CD4 counts, are used to determine the effectiveness of a person’s HIV therapy. When a person begins HIV therapy, a healthcare provider will want to see how well HIV is responding in their body. The goal of HIV therapy is to reduce or suppress the viral load to an undetectable level. According to HIV.gov, HIV viral load is typically undetectable below levels of 40 to 75 copies/mL. The exact number depends on the lab that analyzes the tests.

Blips

Some people may experience blips. These are temporary, oftentimes small increases in viral load. A healthcare provider will monitor the viral load more closely to see if it returns to an undetectable level without any change in therapy.

Drug resistance

Another reason for regular viral load tests is to monitor any drug resistance to the prescribed HIV therapy. Maintaining a low viral load reduces the risk of developing resistance to the therapy. A healthcare provider can use viral load tests to make necessary changes to a person’s HIV therapy regimen.

Why is HIV therapy so important?

HIV therapy is also called antiretroviral therapy or highly active antiretroviral therapy (HAART). It consists of a combination of antiretroviral drugs. They’re designed to keep the virus from spreading throughout your body by targeting different proteins or mechanisms the virus uses to replicate.

Antiretroviral therapy can make the viral load so low that it can’t be detected by a test. This is called an undetectable viral load. If a person is virally suppressed or has an undetectable viral load, their HIV is under control.

Starting HIV therapy as soon as an HIV diagnosis is received allows a person to live a long, healthy life. Current treatment guidelines from the U.S. Department of Health and Human Services recommend that a person living with HIV begin antiretroviral drugs as soon as possible after diagnosis. This is essential to reducing opportunistic infections and preventing complications from HIV.

Another benefit to getting HIV under control and having an undetectable viral load is that it helps prevent the transmission of HIV to others. This is also known as “treatment as prevention.” According to the Centers for Disease Control and Prevention, people with HIV who take their prescribed medications and maintain an undetectable viral load have “effectively no risk” of transmitting HIV to people without it.

What’s the outlook for people with HIV?

No matter the stage of HIV, there are advantages to keeping track of these numbers. HIV treatment has come a long way in recent years. Following a recommended treatment plan and leading a healthy lifestyle can help a person keep their CD4 count high and their viral load low.

Early treatment and effective monitoring can help a person manage their condition, reduce their risk of complications, and live a long and healthy life.

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