What Is a CD4 Count and Why Is It Important?

Measuring a Person's Immune Strength and Response to Therapy

Colorized electromicrograph of HIV-1 virions (in yellow) budding from an infected CD4+ T-cell. Photo Credit: National Institute of Allergies and Infectious Diseases (NIAID)

Question: What is a CD4 count, and why is it important?

Answer: Your immune system contains different types of cells that help protect the body from infection. One of these types of specialized cells are called the CD4 T-cells, which the immune system employs to identify and target infective agents like HIV.

In the course of an HIV infection, the virus preferentially attacks these cells and uses them to make more copies of itself, And in doing so, HIV depletes a key player in the immune response, making the body less and less able to defend itself from illness and infection.

Understanding CD4 Depletion

Early in the course of the disease, the body can activate CD4 T-cells to replace ones that have been damaged by HIV.  But, over time, as the body succumbs the effects of persistent inflammation, it is less able to do so.

As more and more CD4 cells become damaged, the immune system becomes gradually weakened, leaving the body vulnerable to a wide range of associated illnesses and infections.

The CD4 count literally counts the number of CD4 T-cells in a blood sample and tracks whether a person's immune strength is going up or going down, with higher values indicating a  stronger, more robust response.

Along with the HIV viral load, a CD4 count is considered an invaluable diagnostic tool in the care and treatment of people living with HIV.

Measuring the CD4 Count

Generally speaking, a healthy, uninfected person will have a CD4 count of around 700 to 1,500 CD4 cells per mL.

Persons with HIV can also have normal CD4 counts but will almost invariably will experience a gradual drop in CD4 cells if not provided antiretroviral therapy (ART)

If the CD4 count drops below 500 cells/mL, then that person's immune function is said to be suppressed. If it falls below 200 cells/mL, then that person's immune system is considered compromised, with the stage of disease is classified as AIDS.

It is during this stage that the body's immune response is so debilitated as to make it vulnerable to a wide range of opportunistic infections (OIs).

The Value of a CD4 Count

There was a time when the CD4 count was considered the most important test by HIV doctors and was used not only to diagnosed the stage of disease (and likelihood of illness and death) but to direct when ART should be initiated.

Today, this is the less the case. While CD4 counts are central to the determining the prognosis of an HIV-infected individual, they don't always provide a clear-cut picture of the road ahead. It is possible, for example, for a person with a high CD4 count to have a life-threatening OI, while a person with a very low CD4 count may have minimal signs of infection.

Similarly, CD4 recover can also vary enormously and is most often related to the so-called CD4 nadir, or the lowest point to which the CD4 count has dropped. In people with extremely low CD4 nadirs, recovery is typically slow and may not fully recover to "normal" levels. On the other hand, people with higher CD4 nadirs generally recover quite quickly, often within the course of a few months to a year.

Current CD4 Count Recommendations

As such, CD4 counts are no longer used to determine when ART should be initiated.

Current guidance from the World Health Organization and the U.S. Department of Health and Human Services endorses the implementation of HIV therapy at the time of diagnosis, irrespective of CD4 count.

That doesn't mean that CD4 counts are unnecessary. They still provide us our best snapshot of a person's immune strength, as well as his or her response to therapy.  As such, CD4 counts are recommended anywhere from every 3-6 months during early stage treatment and, in some cases, can be monitored every 12 months in persons with sustained viral control.


U.S. Preventive Services Task Force (USPSTF). "Screening for HIV: U.S. Preventive Services Task Force Recommendation Statement." Rockville, Maryland; April 2013; accessed February 7, 2014.

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