CD4 counts are most often used, along with an HIV viral load, to evaluate the immune system of a person diagnosed with human immunodeficiency virus (HIV) infection and to monitor effectiveness of antiretroviral treatment (ART or ARV), also called highly active antiretroviral therapy (HAART).

CD4 cells are types of white blood cells called T lymphocytes or T cells that fight infection and play an important role in immune system function. They are made in the thymus gland and they circulate throughout the body in the blood. (See the "What is being tested?" section for more details.)

CD4 cells are the main target of HIV. The virus enters the cells and uses them to make copies of itself (replicate) and spread throughout the body. HIV kills CD4 cells, so if an HIV infection is not treated, the number of CD4 cells will decrease as HIV infection progresses.

It is recommended that all individuals diagnosed with HIV infection receive antiretroviral treatment as soon as possible, including pregnant women, to reduce the risk of disease progression. People typically take at least three drugs from two different classes in order to prevent or minimize virus replication and the emergence of drug-resistant strains. Combinations of three or more antiretroviral drugs are referred to as highly active antiretroviral therapy or HAART.

Since CD4 cells are usually destroyed more rapidly than other types of lymphocytes and because absolute counts can vary from day to day, it is sometimes useful to look at the number of CD4 cells compared to the total lymphocyte count. The result is expressed as a percentage, i.e., CD4 percent.

The results can tell a health practitioner how strong a person's immune system is and can help predict the risk of complications and debilitating opportunistic infections. CD4 counts are most useful when they are compared with results obtained from earlier tests. They are used in combination with the HIV viral load test, which measures the amount of HIV in the blood, to monitor how effective ART is in suppressing the virus and determine the risk of progression of HIV disease.

Sometimes, CD4 tests may be used along with a test for CD8 cells to help diagnose or monitor other conditions such as lymphoma, organ transplantation, and DiGeorge syndrome (see Common Questions #4). CD8 cells are another type of lymphocyte that identify and kill cells that have been infected with viruses or that have been affected by cancer.

A CD4 count is usually ordered along with an HIV viral load when a person is first diagnosed with HIV infection as part of a baseline measurement. After the baseline, a CD4 count will usually be ordered at intervals over time, depending on a few different factors.

The following table summarizes recommendations* for the timing of CD4 counts and viral load testing:

*Adapted from Guidelines for the Use of Antiretroviral Agents in HIV-1–Infected Adults and Adolescents, Table 4. Recommendations on the Indications and Frequency of Viral Load and CD4 Count Monitoring.

A CD4 count is typically reported as an absolute level or count of cells (expressed as cells per cubic millimeter of blood). A normal CD4 count ranges from 500–1,200 cells/mm³ in adults and teens. Sometimes results are expressed as a percent of total lymphocytes (CD4 percent).

In general, a normal CD4 count means that the person's immune system is not yet affected by HIV infection. A low CD4 count indicates that the person's immune system has been affected by HIV and/or the disease is progressing. However, any single CD4 test result may differ from the last one even though the person's health status has not changed. Usually, a health practitioner will take several CD4 test results into account rather than a single value and will evaluate the pattern of CD4 counts over time.

CD4 counts that rise and/or stabilize over time may indicate that the person is responding to treatment. If someone's CD4 count declines over several months, a health practitioner may recommend starting prophylactic treatment for opportunistic infections such as Pneumocystis carinii (jiroveci) pneumonia (PCP) or candidiasis (thrush).

The CD4 count tends to be lower in the morning and higher in the evening. Acute illnesses, such as pneumonia, influenza, or herpes simplex virus infection, can cause the CD4 count to decline temporarily. Cancer chemotherapy can dramatically lower the CD4 count.

The CD4 count does not always reflect how someone with HIV disease feels and functions. For example, some people with higher counts are ill and have frequent complications, and some people with lower CD4 counts have few medical complications and function well.

The Centers for Disease Control and Prevention (CDC) considers people who have an HIV infection and CD4 counts below 200 cells/mm³ to have AIDS (stage III HIV infection), regardless of whether they have any signs or symptoms.