Difficult Concepts Made Easy

Before you can understand absolute CD4 and CD4 percentage, you have to know some background information.

What are Lymphocytes?

Lymphocytes are a type of white blood cell that helps the body fight infection. There are two main types of lymphocytes; B-lymphocytes and T-lymphocytes. Let's focus on T-lymphocytes, also know as T-cells. There are two types of T-cells:

CD4 Cells - These cells have molecules called CD4 on their surface. They start the immune response that protects the body from infectious invaders such as bacteria and viruses.

CD8 Cells - These cells, with molecules on their surface called CD8, destroy other infected cells and produce antiviral substances that fight off infectious organisms.

What is the absolute CD4 count?

The absolute CD4 count is a measurement of how many functional CD4 T-cells is circulating in the blood.

The lower the absolute CD4 count, the weaker the immune system. The absolute CD4 count is measured by a simple blood test and is reported as the number of CD4 cells per cubic millimeter of blood. HIV-negative people typically have absolute CD4 counts between 600 and 1200 CD4 cells per cubic millimeter. HIV-infected people have counts that are typically less than 500, and people with AIDS can have 200 CD4 cells per cubic millimeter or fewer.

What is the CD4 Percentage?

CD4 percentage represents the percentage of total lymphocytes that are CD4 cells. The CD4 percentage is measured using the same blood test as the absolute CD4 count. Typically, HIV-negative people will have a CD4 percentage of about 40 percent, while HIV-infected people's CD4 percentage can be as low as 25 percent or less. To better understand the concept of CD4 percentage and absolute CD4, let's look at an example.

Say your lymphocytes are 10 apples in a basket. There are two types of apples in the basket; red ones, which represent CD4 cells, and green ones that represent CD8 cells. If five apples are red and five are green, then the CD4 percentage is 50 percent.

Why is it Important to Know the CD4 Count and CD4 Percentage

The CD4 count and CD4 percentage give your doctor a snapshot of the health of your immune system. The immune system needs CD4 cells to function properly. The more CD4 cells in circulation, the stronger the immune system. By knowing the absolute CD4 count, your doctor has an idea of the strength of your immune system.

As early as 1988, infectious disease specialists determined that an absolute CD4 cell count of less than 200 cells per cubic millimeter placed the person at risk for opportunistic infections. In addition to measuring the strength of the immune system, the absolute CD4 count in part determines treatment needs for the patient. If the absolute CD4 count falls below 200, the doctor knows the patient is at risk for opportunistic infections; the doctor may prescribe protective antibiotics for the patient. If the absolute CD4 count rises after starting a new treatment regimen, the doctor knows the medications are effective.

The CD4 percentage provides another clue to the health of the immune system. Because the CD4 percentage takes in consideration the total number of lymphocytes present, many feel the CD4 percentage is a more reliable indicator of immune system health. Let's again take our basket of apples example. Remember, our basket of 10 apples had an absolute CD4 count of five and a CD4 percentage of 50 percent. Say the basket now has 25 apples, five red and 20 green. The absolute CD4 count is the same - five. However, the CD4 percentage in the second example is only 20 percent (five apples out of 25 are red), indicating a much weaker immune system.

CD4 percentage takes in consideration factors which could cause a false high or false low CD4 count. If the absolute CD4 count is higher only because the total lymphocyte count is higher that does not indicate the immune system is stronger, only that there are more lymphocytes contributing to the absolute CD4 count. In our example, the absolute CD4 count of five didn’t mean as much in the basket of 25 apples as it did in the basket of 10. The count was five only because there were more apples.

There is some debate as to which measurement is a better predictor of opportunistic infection risk. In some situations, CD4 percentage seems to be as good a predictor as absolute CD4 counts. A recent study by D.M. Moore and his colleagues, published in HIV Medicine in September 2006, showed that in the case of people with no HIV treatment experience, absolute CD4 count and CD4 percentage both could predict HIV progression. However, the same study found that when the CD4 percentage is less than 15 percent, CD4 percentage should be considered along with the absolute CD4 count when determining illness risk and when to start HIV treatment. Finally, a study due out in the February issue of Journal of Infectious Diseases suggests CD4 percentage is a better predictor of disease progression.

Let’s Summarize

Absolute CD4 count – the number of function CD4 cells circulating in the patient’s blood. The absolute CD4 count does not take the total lymphocyte count into consideration when assessing immune system health.

CD4 Percentage – the percentage of all functional lymphocytes that are CD4 cells. The CD4 percentage takes in consideration the total lymphocyte count when assessing immune system health.

What Does All This Mean to the Patient

CD4 percentage and absolute CD4 count are complex concepts. If patients can understand what CD4 percentage and absolute CD4 count are and how they differ, the patient can better understand the decisions made by their physician. In addition, understanding the two concepts helps the patient to participate in his or her own HIV care.

Gebo, K. "Absolute CD4 versus CD4 Percentage for Predicting the Risk of Opportunistic Illness in HIV Infection." Journal of Acquired Immune Deficiency Syndromes. 25 Aug 2004.

Moore, D.M. "CD4 Percentage is an Independent Predictor of Survival in Patients Starting Antiretroviral Therapy with Absolute Cell Counts between 200 and 350 cells/cubic millimeter."HIV Medicine. 13 Sep 2006.

Updated: January 21, 2007

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