Blood-tests Explained


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This website is intended to assist with patient education and should not be used as a diagnostic, treatment or prescription service, forum or platform. Always consult your own healthcare practitioner for a more personalised and detailed opinion

CD4 Count


At a Glance

Why Get Tested?

Most often, this test is done to measure the strength of your immune system if you have been diagnosed with human immunodeficiency virus (HIV) infection and to monitor treatment; occasionally, it may be used with other conditions (see Common Questions #4).

When to Get Tested?

When you are first diagnosed with HIV infection to get a baseline assessment of your immune system; about every 3 to 6 months after starting antiretroviral therapy (ART) to check whether you are responding to treatment; if you have responded well to treatment, you may then be tested every 6 to 12 months.

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?

None

The Test Sample

What is being tested?

CD4 cells are 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 and lymphatic system. CD4 tests measure the number of these cells in the blood and, in conjunction with an HIV viral load test, help assess the status of the immune system in a person who has been diagnosed with human immunodeficiency virus (HIV) infection.

CD4 cells are so called because they have markers on their surfaces called clusters of differentiation (CD). The CD number identifies the specific type of cell.

CD4 cells are sometimes called T-helper cells. They help to identify, attack, and destroy specific bacteria, fungi, and viruses that cause infections. CD4 cells are a major target for HIV, which binds to the surface of CD4 cells, enters them, and either replicates immediately, killing the cells in the process, or remains in a resting state, replicating later.

If HIV goes untreated, the virus gets into the cells and replicates, the viral load increases, and the number of CD4 cells in the blood gradually declines. The CD4 count decreases as the disease progresses. If still untreated, this process may continue for several years until the number of CD4 cells drops to a low enough level that symptoms associated with AIDS begin to appear.

Treatment for HIV infection, called antiretroviral treatment (ART or ARV) or sometimes highly active antiretroviral therapy (HAART), typically involves taking a combination of drugs. This treatment reduces the amount of HIV (viral load) present in the body and reduces the risk of disease progression. When this occurs, the CD4 count will increase and/or stabilize.

CD8 cells are another type of lymphocyte. They are sometimes called T-suppressor cells or cytotoxic T cells. CD8 cells identify and kill cells that have been infected with viruses or that have been affected by cancer. They play an important role in the immune response to HIV infection by killing cells infected with the virus and by producing substances that block HIV replication.

As HIV disease progresses, the number of CD4 cells will decrease in relation to the number of total lymphocytes and CD8 cells. To provide a clearer picture of the condition of the immune system, test results may be reported as a ratio of CD4 to total lymphocytes (percentage).

CD4 and CD8 tests may be used occasionally in other conditions, such as lymphomas and organ transplantation (see Common Questions #4).

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

The Test