The human immunodeficiency virus, also known as HIV infection, is a retrovirus that causes AIDS (acquired immune deficiency syndrome). The retrovirus primarily attacks the body's immune system, making the patient extremely vulnerable to opportunistic infections (infections that occur in immunocompromised individuals).
HIV is transmitted from person to person via bodily fluids, including blood semen, vaginal discharge, and breast milk.
HIV can infect and kill many different types of cells in the body, but the primary targets are immune cells called CD4 T-cells. The CD4 T-cells are a type of T-lymphocyte (white blood cells) that helps coordinate the immune system's response to infection and disease. These cells express a molecule called CD4 on their surfaces, which allow them to detect foreign substances, including viruses that enter the body. HIV binds to the receptors on CD4 cells and enters the white blood cell. Once inside the cell, HIV begins replicating.
The first stage of HIV, known as the primary or acute infection, is the most infectious stage of the disease, and it typically lasts several weeks. During this phase, the virus replicates rapidly, which leads to an abundance of the virus in the bloodstream and a drastic decline in the number of CD4 T-cells.
The CD8 T-cells (cells that kill abnormal or infected body cells) are then activated to destroy HIV-infected body cells and antibodies are produced. An estimated 80-90% of HIV patients experience flu-like symptoms during this stage.
The next stage, called clinical latency, may last anywhere from two weeks to 20 years. During this phase, HIV is active in the lymph nodes, where large amounts of the virus become trapped. The surrounding tissues, which contain high levels of CD4 T-cells, may also become infected. The virus accumulates in infected cells and in the blood as free virus.
Patients progress to AIDS when their CD4 cell counts drop below 200 cells per microliter of blood. Healthy individuals have a CD4 cell count between 600 and 1,200 cells per microliter of blood. Individuals with a CD4 cell lower than 200 cells per microliter of blood have the greatest risk of developing opportunistic infections.
Several different CD4 tests are used along with a viral load test to evaluate HIV/AIDS patients' medical conditions and monitor their responses to treatment. CD4 blood tests measure the amount of CD4 T-cells that are circulating in the blood, while viral load tests determine how many viral particles are present in the blood.
HIV patients who are otherwise healthy and symptom-free should have their CD4 cell count and viral load tested about two to four times a year However, symptomatic patients should be tested more frequently to evaluate both the risk of opportunistic infections and the response to HIV drug treatments.
The absolute CD4 cell count test is a blood test that measures the number of CD4 T-cells in each microliter of blood. Healthy adults who are HIV-negative typically have 600-1,200 CD4 cells per cubic millimeter of blood. HIV-positive patients have lower counts because the virus infects and destroys these cells. The lower the count, the greater the risk of infections. Patients progress to AIDS (acquired immune deficiency syndrome) when their CD4 cell counts drops below 200 cells per microliter of blood. Individuals who have CD4 cell counts below 200 are at the greatest risk of developing opportunistic infections because there is no longer cell-mediated immunity.
This is the standard test performed to monitor the body's immune system. Healthcare providers can use the test results to determine if preventative treatment or changes in antiretroviral therapy are necessary to prevent infections.
A CD4 percentage test is a blood test that measures the proportion of CD4 cells as a subset of all lymphocytes in the blood. These tests may be performed alone or in addition to absolute CD4 tests.
In HIV-negative people, about 40% of lymphocytes are CD4 cells. HIV patients will have markedly lower counts. A CD4 percentage lower than 20% reflects a high risk of opportunistic infections, which is about the same as an absolute CD4 cell count of about 200 cells per microliter of blood.
Unlike an absolute CD4 cell count, a CD4 percentage test takes into consideration factors that could cause a false high absolute CD4 count. For instance, if the absolute CD4 count is higher because the total lymphocyte count is higher, it does not mean that the immune system is stronger.
Experts debate whether a CD4 percentage test or absolute count is a better indicator of opportunistic infection risk. According to a 2006 study, percentage CD4 tests and absolute CD4 count tests were able to predict HIV progression equally well. However, researchers from the same study found that when the CD4 percentage is less than 15%, the results should be considered along with the absolute CD4 count in order to accurately determine the risk of infection and to plan appropriate treatment options.
A CD4:CD8 ratio test may also be performed to determine how damaged the immune system is. During this test, the blood analysis shows the number of CD4 cells in a sample of blood compared with the number of CD8 cells. The result is reported as a single number, which indicates how many CD4 cells are present for each CD8 cell. HIV-negative people will have a result greater than one, which indicates that there is at least one CD4 cell for each CD8 cell. HIV patients will have values below one.
The results of this test fluctuate during the different stages of HIV because the rate of decline is generally proportional to the patient's viral load. The higher the viral load, the lower the CD4:CD8 ratio. This test is typically used to determine the risk of disease progression.
A viral load test measures the number of copies of HIV in one milliliter of blood. This test is used in conjunction with CD4 testing to assess HIV progression and monitor the efficacy of antiretroviral treatment.
If an HIV-positive patient is not treated, HIV can produce billions of new viral copies each day.
Viral load tests count up to about one million copies. HIV patients who are otherwise healthy and receiving antiretroviral therapy (ART) usually have about 200-500 copies per milliliter of blood. This indicates that the virus is not actively replicating, and the risk of progressing to AIDS is low. Some HIV patients who are receiving ART may have undetectable levels of the virus. This does not mean that the patient is cured. It simply means that the viral load is lower than the test's threshold for detection. Some tests can measure as few as 20-40 copies per milliliter of blood. A high viral load can be anywhere from 5,000-1,000,000 or more copies. A high viral load indicates that HIV is replicating and the disease will most likely progress quicker than if the viral load is low.
According to treatment guidelines in the United States, anyone who has a viral load higher than 100,000 copies per milliliter of blood should be offered treatment.
The test is typically performed at least twice before beginning antiretroviral therapy. These results serve as a baseline and they help healthcare providers determine how actively the virus is replicating and whether the patient is responding to treatment. Patients are also tested two to eight weeks after antiretroviral therapy (ART) is started or changed. It should then be measured every three to four months to monitor whether the treatment is effective. If the drugs are effective, they will lower the viral load by at least 90% within eight weeks. After six months of successful treatment, the viral load should decline to less than 50 copies.
However, the viral load tests only measures the amount of viral particles present in the blood. Only about two percent of the HIV is present in the blood. The rest of the viral particles are present inside body cells and tissues, including the spleen, brain, and lymph nodes. The viral load test results can be incorrect if the immune system is fighting an infection or if the patient has just received an immunization. Therefore, patients should not have viral load test within four weeks of any infection or immunization.
Antiretroviral therapy (ART) is designed to increase the patient's CD4 cell count by suppressing HIV. The drugs interfere with HIV's ability to replicate and infect new cells, which slows down the progression of the disease.
CD4 cell count tests and viral load tests can indicate whether or not the treatment is working. In response to successful antiretroviral therapy (ART), the CD4 count typically increases by more than 50 cells per microliter of blood within a few weeks and then increases by 50-100 cells per year thereafter until a threshold is reached. Thresholds vary among patients. Antiretroviral therapy (ART) is taken indefinitely to control the virus.
These tests help healthcare providers determine whether the treatment needs to be changed. The tests also indicate when preventative treatment against opportunistic infections may be necessary. For instance, when the absolute CD4 cell count drops below 200 cells per microliter of blood, preventative antibiotics may be prescribed for potentially life-threatening infections.
The patient should be not be evaluated on the basis of a single test. Instead, the physician should look at the results from a series of tests, such as an absolute CD4 count, CD4 percentage test, CD4:CD8 ratio and viral load test, because many factors can influence the test results. For instance, other infections, the time of day, stress levels, and whether or not the patient smokes can all an impact each of the test results.
In addition, an absolute CD4 test only indicates the number of CD4 cells that are present in the blood. However, most CD4 cells are present in the body tissues, including the lymph nodes. Many factors, such as an infection, can encourage CD4 cells to move into or out of the blood, which can alter the results of the test. This is why other tests are commended in conjunction with an absolute CD4 test.
Testing frequency depends on a patient's overall health and whether or not he/she is receiving antiretroviral therapy. Untreated patients who have CD4 cell counts higher than 500 are usually tested once every six to 12 months. Individuals who have CD4 counts between 350 and 500 are usually tested about once every three to six months. Patients who have CD4 cell counts between 250 and 350 may need to be tested more frequently so that preventative treatment against opportunistic infections can be administered quickly if the count drops below 200.
Healthcare providers usually recommend a CD4 cell count test before starting antiretroviral therapy. A second test is usually conducted four to eight weeks after the initiation of treatment to assess the effectiveness of the drugs.
A viral load test is typically performed at least twice before beginning ART. These results serve as a baseline, and they help healthcare providers determine how actively the virus is replicating, and whether the patient is responding to treatment. Patients are also tested two to eight weeks after ART is started or changed. It should then be measured every three to four months after that. This helps a healthcare provider determine whether the treatment is effective. If the drugs are effective, it will lower the viral load by at least 90% within eight weeks. After six months, the viral load should decline to less than 50 copies.
The information in this monograph is intended for informational purposes only, and is meant to help users better understand health concerns. Information is based on review of scientific research data, historical practice patterns, and clinical experience. This information should not be interpreted as specific medical advice. Users should consult with a qualified healthcare provider for specific questions regarding therapies, diagnosis and/or health conditions, prior to making therapeutic decisions.