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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 individuals not infected with HIV, the CD4 count in the blood is normally above 500 cells per cubic milliliter (mm3) of blood. HIV-infected people generally do not become at risk for 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 suppressed. Patients who have this CD4 count (fewer than 200 cells per mm3) are referred to as being immunosuppressed. A declining number of CD4 cells means that the HIV disease is advancing. Thus, a low CD4 cell count signals that the person is at risk for one of the many unusual infections (the so-called 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 predicts whether or not the CD4 cells will decline in the coming months. In other words, those persons with high viral loads are more likely to experience a decline in CD4 cells and progression of disease than those with lower viral loads. Therefore, knowing the amount of virus can be used to predict the development of the disease. The viral load also is a vital tool for monitoring the effectiveness of new therapies and determining when drugs stop 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 100-fold, such as from 100,000 to 1,000 copies per mL of blood in the first 2 weeks and gradually decrease even further during the ensuing 12 to 24 weeks. Moreover, it has become increasingly clear that the greater the decline of the viral load after beginning therapy, the longer it will remain suppressed. The ultimate goal is to get viral loads to below the limits of detection by standard assays, usually less than 50 or 75 copies per mL of blood. When viral loads are reduced to these low levels, it is believed that the viral suppression may persist for many years.
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 100-fold in the first 8 weeks, have a viral load of greater than 500 copies per mL by week 12, or have levels greater than 50 or 75 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. More recent guidelines from the U.S. Department of Health and Human Services (DHHS) (www.hivatis.org) and International AIDS Society-USA (IAS-USA) have suggested that resistance testing be considered in individuals who have never been on therapy, particularly in the first months or even years of infection, to determine if they might have acquired HIV that is resistant to drugs. In fact, the most recent DHHS guidelines (May 4, 2006) formally recommend such testing be performed in all individuals starting therapy for the first time.