Testing for Patients with HIV/AIDS
HIV is a life-long infection that requires continuous monitoring and treatment. Monitoring HIV infection and its progression to AIDS involves the following types of tests:
- Viral load and T-cell/CD4+ counts measure how much HIV is in the blood and help determine how quickly HIV/AIDS is progressing. These tests are used as guidelines for when to begin and change antiretroviral therapy.
- Resistance testing indicates if HIV has developed resistance to a particular antiretroviral drug. This test helps the physician decide which antiretroviral medications to prescribe and when to change medication.
- Blood tests are done to evaluate the presence of HIV-related opportunistic infections.
A viral load test provides a measure of the amount of HIV in the blood, specifically the number of copies of viral RNA per one milliliter (mL or cc) of blood. Viral loads in patients infected with HIV can range from undetectable to more than a million copies per mL. These tests are not sensitive enough to detect viral loads lower than about 25 copies per mL. Therefore, even though an undetectable viral load is a very good sign, it does not mean that the patient is HIV-negative. It just means that there is not enough virus in the blood to be detected.
There are number of viral load tests available and patients should discuss the options with their health care provider. It is important that patients stick with one kind of test over time to be sure that changes in the viral load test reflect actual changes in the blood and not differences between the tests.
The American Medical Association guidelines for viral testing are as follows:
- Two tests should be performed within 1 or 2 weeks of each other to establish what is known as a baseline viral count. The baseline count provides an initial measure that the physician can use for monitoring the progression of HIV/AIDS. It is recommended that patients who have advanced HIV/AIDS disease be treated with antiretrovirals immediately after the first test in order to avoid potentially damaging delays.
- After the baseline viral count is measured, tests should be repeated every 3 to 6 months, along with T-cell/CD4 counts.
- Tests should be repeated 4 to 8 weeks after beginning or changing antiretroviral therapy.
Scientists have found a correlation between viral load and disease progression—the higher the viral load, the farther the infection has progressed, even if there are no symptoms.
Viral load helps the physician evaluate antiretroviral treatment and determine when to change medications. A higher viral load is an indication that treatment should be adjusted.
Recent infection or immunization can affect viral load assay results. Patients should wait several weeks following infection or immunization to have their viral load measured.
T-cell Test (CD4+ Count)
The T-cell test is usually reported as the number of T-cells, also known as CD4+ cells, in one milliliter (mL) of blood. Healthy, uninfected people have between 500 and 1600 CD4+ cells per mL of blood. According to the Centers for Disease Control and Prevention (CDC), a CD4+ cell count below 200/mL is a criterion for AIDS.
T-cell tests also are used to measure of a different kind of immune system cell, known as a CD8+ cell. Sometimes, the ratio of CD4+ to CD8+ cells is used to monitor HIV infection, since the ratio drops so dramatically in patients with HIV/AIDS. In healthy people, there are normally about 1 to 2 CD4+ cells for every CD8+ cell. In patients with HIV/AIDS, the ratio is reversed and there are many CD8+ cells for every CD4+ cell.
T-cell levels depend on a variety of factors (e.g., stress, fatigue) and can change considerably throughout the day. If possible, patients should have the test at the same time of day each time.
The number of CD4+ and CD8+ cells increases during an infection. So the results of T-cell tests can vary depending on whether the patient has had any recent infections.
Physicians use viral loads and T-cell/CD4+ counts as approximate guides for starting treatment. There are no magic numbers, but the International AIDS Society recommends antiretroviral therapy when:
- The patient has a symptomatic HIV infection, no matter what the test results are
- The viral load rises above about 30,000 copies/mL, no matter what the T-cell/CD4+ count is
- The T-cell/CD4+ counts fall below 350 x 106/L (350/ul), no matter what the viral load is
- The viral load is between 5000 and 30,000 copies/mL and the T-cell/CD4+ cell count is between 350 and 500 x 106/L
Patients with a viral load below 5000 copies/mL and a T-cell/CD4+ count above 500 x 106/L are at low risk for short-term (3 year) clinical progression of disease. Whether therapy should be started or not in these patients depends on potential side effects, the patient's willingness to adhere to the strict therapy regimen, and other factors such as health insurance coverage.
When antiretroviral drugs are effective, the viral load should decrease to an undetectable level within 30 to 90 days of initial treatment. The T-cell/CD4+ count should simultaneously increase, usually by about 100 cells/cc during the first year.
Resistance testing evaluates whether HIV has evolved resistance to particular drugs. This test can help physicians decide on an initial antiretroviral regimen and help guide choices when treatment must be changed.
There are two types of drug resistance tests:
- Genotype assays examine the HIV's genetic material and identify particular mutations responsible for resistance. Generally, the viral load must be above a certain value (i.e., greater than 1000 copies/mL) for genotype testing to be useful. If genotype testing reveals a mutation known to cause resistance to a particular drug, the patient's drug therapy must be managed accordingly.
- Phenotype assays measure the ability of the virus to grow and replicate in the presence of varying concentrations of drugs. A virus that grows well in the presence of even a high concentration of drug indicates resistance.
There are many different types of both genotype and phenotype assays, many of which still need to be clinically tested and adequately standardized before becoming commercially available.
Despite the reliability of these tests, problems with drug resistance assays include the following:
- They are expensive.
- Turnaround time is usually several weeks, which is too slow for treating the fast-evolving HIV.
- Negative results are difficult to interpret. The assays cannot detect all resistant genotypes or phenotypes.
As of June 2000, the American Medical Association recommends resistance testing to help determine a patient's initial antiretroviral regimen only if there are factors that indicate an increased risk for resistance.
For example, there is some concern that HIV in women who are pregnant and in newborn children is more likely to develop resistance than in the general HIV-positive population. Patients should consult with their physicians to find out if they are at high risk for resistance, and if so, how resistance testing may benefit them.
The American Medical Association cautions that decisions to change therapy should be based primarily on a confirmed increase in viral load and the patient's treatment history. Resistance testing should be considered only as an additional source of information that may or may not be helpful.