CD4 and CD8
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The CD4 and CD8 test is performed on a blood sample to measure the number and proportion of CD4 and CD8 T‑lymphocytes, which are key cells in the immune system. It is used mainly to assess immune system function and monitor people with HIV infection, often alongside the HIV viral load test.
Why get tested?
Most often, this test is done to measure the strength of your immune system if you’ve been diagnosed with HIV infection. Occasionally it may be used with other conditions.
When to get tested?
If you’ve been diagnosed with HIV, soon after you are first diagnosed to get a baseline assessment of your immune system; 2–8 weeks after starting anti-HIV therapy and then every three to four months if you continue therapy
Sample required?
A blood sample taken from a vein in your arm
Test preparation needed?
No test preparation is needed
What is being tested?
CD4 and CD8 cells are lymphocytes that have markers on the surfaces of the cells called CD4 and CD8. They are types of white blood cells that fight infection, and they play an important role in your immune system function. CD4 and CD8 cells are made in the bone marrow, and mature in the thymus gland, a small gland found in the upper chest. They circulate throughout the body in the bloodstream, spleen and the lymph nodes.
CD4 cells are sometimes called T‑helper cells. They help to identify and trigger the attack and destruction of specific bacteria, fungi and viruses that affect the body. 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. As the HIV virus gets into the cells and replicates, the number of CD4 cells in the blood gradually declines. The CD4 count decreases with HIV disease progression. This process may continue for several years before the number of CD4 cells drops to a low enough level that symptoms associated with AIDS begin to appear. As treatment reduces the amount of HIV present in the body and slows progression, the CD4 count will increase and/or stabilize.
CD8 cells include lymphocytes that are called cytotoxic T cells and T‑suppressor cells. CD8 cytotoxic T‑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 by killing cells infected with the virus and by producing substances that block HIV replication.
These tests measure the number of CD4 and CD8 cells in the blood and, in conjunction with an HIV viral load test, assess the status of the immune system if you have been diagnosed with HIV. As the 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, the results of these tests may be expressed as a ratio of CD4 to total lymphocytes (percentage) or as a ratio of CD4 cells to CD8 cells.
These tests are most often used to help monitor disease progression in HIV but may also be used occasionally in other conditions such as lymphomas and patients receiving immune suppresive medications or in the diagnosis and monitoring of primary and non-HIV related immune deficiences. (See FAQ #4)
Common questions
If you have been diagnosed with HIV, a CD4 count is used to help evaluate and track the progression of HIV infection and disease. CD4 cells are the main target of HIV and the number of CD4 cells will decrease as HIV progresses. 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 other types of lymphocytes. Sometimes a CD4 cell count is compared to the total lymphocyte count and the result is expressed as a percentage, or the CD4 cell count may be compared to the CD8 cell count, and the result is expressed as a ratio. However the absolute CD4 T‑cell count is the most important value.
The CD4 count and CD4/CD8 ratio can tell your health care team how strong your immune system is and can help predict the risk of complications and debilitating infections. These tests are most useful when they are compared with results obtained from earlier CD4 counts. They are used in combination with the HIV viral load test, which measures the level of HIV in the blood, to determine the progression and outlook of HIV disease and to monitor the effectiveness of treatment.
A normal CD4 count does not exclude HIV and an abnormal CD4 count can be found for many other reasons, so a CD4 count should not be performed as a ‘surrogate’ test for HIV.
Sometimes these tests may be used to help diagnose or monitor other conditions such as lymphoma, primary immune deficiency, and in patients receiving immune suppression (e.g. for transplantation). (See FAQ #4).
A CD4 count, or sometimes a CD4/CD8 ratio is ordered with a viral load test when you are first diagnosed with HIV as part of a baseline measurement. The tests should be repeated about two to eight weeks after starting or changing anti-HIV therapy. If treatment is maintained, they should be performed about every three to four months thereafter.
In patients with severe, recurrent, atypical or invasive infection who are known to be HIV negative a CD4 and CD8 count may be requested as part of an assessment of immune function.
Patients receiving immune suppression or being treated for blood cell cancers e.g. lymphoma may have the CD4 and CD8 count measured as part of assessment of the immune system and treatment.
A CD4 count may be interpreted as an absolute level, as a ratio to CD8 or as a percent of total lymphocytes. In adults, the CD4 absolute value or count is used to guide treatment decisions. In children, the normal ranges change significantly from birth to teenage years and the percent CD4 is usually used as a guide to treatment need or response.
In general, the CD4 count and percentage goes down as HIV disease progresses. Any single CD4 count value may differ from the last one even though your health status has not changed. Your doctor will take several CD4 test results into account rather than a single value and will evaluate the pattern of CD4 counts over time.
If your CD4 count declines over several months, your health care team may recommend beginning or changing anti-retroviral treatment and/or starting prophylactic treatment for opportunistic infections such as Pneumocystis pneumonia (PCP) or Mycobacterum avium infection. Your CD4 count should increase or stabilize in response to effective combination anti-HIV therapy.
According to the BHIVA 2015 (interim 2016 update) guidelines antiretroviral therapy should be started in all adult patients diagnosed with HIV irrespective of CD4 count. For Children the PENTA guidelines (2015) recommend all children under 1 year of age and thereafter a CD4 percent and absolute based guidance if there is no clinical evidence of disease. The adult guidelines have changed significantly in the last 5 years and the paediatric guidance is likely to continue to be updated.
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.
A 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 counts have few medical complications and function well.
HIV is usually screened for and diagnosed with an HIV antibody test about 2–8 weeks after exposure to the virus. If exposure is more recent, a p24 antigen test or an HIV viral load test may be used to detect the virus in your blood.
You and your health care team should discuss your treatment options to determine what will work best for you.
Yes. Evaluation of CD4 and CD8 cells may also be used to help classify lymphomas. Typically, several lymphocyte surface markers in addition to CD4 and CD8 are evaluated. The tests help determine whether the lymphoma is due to the proliferation of B cells or T cells and which specific type. This information is useful in determining appropriate therapy.
These tests may also help diagnose rarer genetic immune deficiency disorders characterized by, among other things, low levels of T cells in the blood. For more information on primary immunodeficiency syndromes, visit the Immune Deficiency Foundation website. Patients receiving immune suppression for a variety of conditions may have a CD4 and CD8 count measured to check response to treatment or their susceptibility to infection.