Human Immunodeficiency Virus (HIV) (cont.)
Eric S. Daar, MD
Eric S. Daar, MD
Dr. Daar received his undergraduate degree from UCLA and medical degree from Georgetown University School of Medicine. He completed an internship and residency in internal medicine at Cedars-Sinai Medical Center and his clinical and research fellowship in infectious diseases at Cedars-Sinai Medical Center and UCLA.
In this Article
What happens after an exposure to the blood or genital secretions of an HIV-infected person?
The risk of HIV transmission occurring after any potential exposure to bodily fluids is poorly defined. The highest risk sexual activity, however, is thought to be receptive anal intercourse without a condom. In this case, the risk of infection may be as high as 3%-5% for each exposure. The risk is probably less for receptive vaginal intercourse without a condom and even less for oral sex without a latex barrier. Despite the fact that no single sexual exposure carries a high risk of contagion, HIV infection can occur after even one sexual event. Thus, people must always be diligent in protecting themselves from potential infection.
During all stages of infection, literally billions of HIV particles (copies) are produced every day and circulate in the blood. This production of virus is associated with a decline (at an inconsistent rate) in the number of CD4 cells in the blood over the ensuing years. Although the precise mechanism by which HIV infection results in CD4 cell decline is not known, it probably results from a direct effect of the virus on the cell as well as the body's attempt to clear these infected cells from the system. In addition to virus in the blood, there is also virus throughout the body, especially in the lymph nodes, brain, and genital secretions.
What laboratory tests are used to monitor HIV-infected people?
Two blood tests are routinely used to monitor HIV-infected people. One of these tests, which counts the number of CD4 cells, assesses the status of the immune system. The other test, which determines the so-called viral load, directly measures the amount of virus in the blood.
In individuals not infected with HIV, the CD4 count in the blood is normally above 400 cells per mm3 of blood. People generally do not become at risk for HIV-specific complications until their CD4 cells are fewer than 200 cells per mm3. At this level of CD4 cells, the immune system does not function adequately and is considered severely suppressed. A declining number of CD4 cells means that HIV disease is advancing. Thus, a low CD4 cell count signals that the person is at risk for one of the many opportunistic infections that occur in individuals who are immunosuppressed. In addition, the actual CD4 cell count indicates which specific therapies should be initiated to prevent those infections.
The viral load actually measures the amount of virus in the blood and may partially predict whether or not the CD4 cells will decline in the coming months. In other words, those people with high viral loads are more likely to experience a decline in CD4 cells and progression of disease than those with lower viral loads. In addition, the viral load is a vital tool for monitoring the effectiveness of new therapies and determining when drugs are and are not working. Thus, the viral load will decrease within weeks of initiating an effective antiviral regimen. If a combination of drugs is very potent, the number of HIV copies in the blood will decrease by as much as hundredfold, such as from 100,000 to 1,000 copies per mL of blood in the first two weeks and gradually decrease even further during the ensuing 12-24 weeks. The ultimate goal is to get viral loads to below the limits of detection by standard assays, usually less than 20 to 50 copies per mL of blood. When viral loads are reduced to these low levels, it is believed that the viral suppression will persist for many years as long as the patient consistently takes their medications.
Drug-resistance testing also has become a key tool in the management of HIV-infected individuals. Details of these tests will be discussed later. Clearly, resistance testing is now routinely used in individuals experiencing poor responses to HIV therapy or treatment failure. In general, a poor response to initial treatment would include individuals who fail to experience a decline in viral load of approximately hundredfold in the first weeks, have a viral load of greater than 500 copies per mL by week 12, or have levels greater than 50 copies per mL by week 24. Treatment failure would generally be defined as an increase in viral load after an initial decline in a person who is believed to be consistently taking his or her medications. Since drug-resistant virus can be transmitted, guidelines from the U.S. Department of Health and Human Services (DHHS) (http://www.aidsinfo.nih.gov) and International Antiviral Society-USA (IAS-USA) have suggested that resistance testing be performed in individuals who have never been on therapy to determine if they might have acquired HIV that is resistant to drugs.
Medically Reviewed by a Doctor on 12/17/2015
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