CD4 and CD8 are two types of white blood cells in your blood. CD4 cells are also called T-helper cells, T-suppressor cells, and cytotoxic T-cells. They help the body fight infections. CD8 cells are also called cytotoxic T-lymphocytes. They help fight cancer and germs that live inside your cells (intracellular pathogens). The human immunodeficiency virus (HIV) chiefly targets CD4 cells. A reduced number of these cells and a change in the CD4:CD8 ratio are signs of HIV infection. The CD4:CD8 ratio helps your physician judge disease activity and your response to treatment.
CD4 Cell Count
The CD4 cell count is a good measure of the activity of the HIV infection and the success of the treatment. Normal CD4 numbers in the blood range between 500 to 1200 per cubic millimeter. The normal range for CD8 is 150 to 1000 per cubic millimeter. A CD4:CD8 ratio of more than 1.0 is considered normal. A CD4 count below 200 per cubic millimeters shows that you have acquired immunodeficiency syndrome (AIDS). Low CD4 counts are also seen in bone marrow suppression because of chemotherapy, long-standing infections, and some types of anemia.
Once you start ART (antiretroviral therapy, a combination of antiviral medicines), your CD4 cell count increases gradually. Your physician will get your CD4 count tested every 3 to 6 months. The return of the CD4 count to normal levels (500 per cubic millimeter) is a sign that the HIV infection is under control.
Traditionally, CD4 counts and viral loads were used to judge the body's response to HIV treatment. But you may be experiencing a weakening of your immune system despite having a normal CD4 count. The CD4:CD8 ratio is more useful in judging the immune system's function. People with HIV and a low CD4:CD8 ratio have increased frailty and mortality.
The total CD4 number, their proportion (CD4 percentage), and their ratio to CD8 cells (CD4:CD8 ratio) are tests used to guide HIV treatment. A CD4:CD8 ratio of more than 1.0 is considered normal. A low ratio (more CD8 cells than CD4) is often seen in HIV infections.
The CD4:CD8 ratio test judges the strength of your immune system. It also tells your physician if you are at risk for opportunistic infections like pneumocystis pneumonia. Your physician may want to protect you by prescribing some medicines.
A low CD4:CD8 ratio during treatment predicts that the virus hasn't been completely removed from your blood. If treatment begins early in the course of an HIV infection, your CD4:CD8 ratio is likely to become normal in about 6 years. Late and irregular treatments are associated with this test remaining abnormal.
The currently used antiretroviral drugs are very effective. Viral loads become undetectable in the blood, and the CD4 count returns to normal with some years of treatment. But several complications of AIDS still happen. The CD4:CD8 ratio is more accurate at predicting the risk of AIDS-associated cancers such as Kaposi's sarcoma and non-Hodgkins lymphoma.
Low CD4:CD8 Ratio
This test is most often done to assess HIV infection status, but it also helps in the diagnosis and monitoring of other diseases:
- Infectious mononucleosis
- Chronic lymphocytic leukemia
- Hodgkin disease (lymphoma)
- Aplastic anemia
- Multiple sclerosis
- Myasthenia gravis
- DiGeorge syndrome
- Organ transplantation
High CD4/CD8 Ratio
The CD4:CD8 ratio can be high in:
- Major bacterial infection
- Viral infection
- Some types of blood cancer
Testing for HIV
HIV is the most frequent reason for abnormal CD4:CD8 ratios. But this test should not be used to diagnose an HIV infection. Your physician will ask for antigen, antibody, or nucleic acid tests to diagnose an HIV infection.
Once your physician has diagnosed HIV, they will start treatment. The CD4:CD8 ratio test and the CD4 cell count are helpful in assessing the progress of the infection. Your physician uses these tests to know if your infection is under control and whether you are at risk of complications.
Another way to assess the progress of the HIV infection is to test the viral load. This measures the amount of HIV in your blood. A high viral load means that your immune system is not fighting the virus well. Your physician will test your viral load every 4 to 6 months, and will also test it if they change your medicines.
High CD8 Counts
Your doctor monitors CD8 counts frequently while you are on treatment for HIV. If your CD8 count is high, it means your viral load is low. Your CD4 count is likely to return to normal levels. CD8 cells are important in the body's defense against HIV, and high CD8 count means that your body is effectively controlling the infection.
If your CD8 counts have been low or normal for some time and then become high, your physician is warned that treatment failure is about to happen. The CD8 cells increase in response to increasing viral load. Treatment failures happen in 20% to 40% of people on ART. Your doctor will ask if you're regular about taking your medicines, and consider other causes of treatment failure.
HIV infection was a deadly disease a few decades ago. People with untreated AIDS usually die within three years. With treatment and proper monitoring, people with HIV can live long and productive lives. CD4 counts, viral loads, and the CD4:CD8 ratio tests help your physician judge your disease control and fine-tune your treatment. Getting these tests done regularly will help you keep the infection under control.