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Understanding HIV-Related Lab Tests II: Viral Load, Resistance and Tropism

July 2013

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Resistance Test

HIV drugs are designed to keep your viral load under control by preventing the virus from reproducing (making copies of itself). Sometimes HIV changes, or mutates as it makes copies of itself. These changes may allow the virus to overcome the effects of a drug and keep reproducing. When this happens, we say that HIV has developed resistance to that drug.

Resistance decreases the ability of HIV drugs to control your virus and reduces your treatment options. The best way to prevent resistance is to stick closely (adhere) to your HIV drug regimen. With good adherence, resistance is less likely to develop.

Drug resistance tests are used to determine if your virus has developed resistance to HIV drugs. There are several types of resistance tests available.

  • Genotype test
    This test uses HIV from your blood to check for mutations -- changes in the virus's genetic sequence or genetic code -- associated with drug resistance
  • Phenotype test
    This test exposes your virus to many HIV drugs in a test tube to determine which ones still work against your HIV
  • Virtual phenotype test
    This is a genotype test that goes one step further -- it uses phenotype information to predict which drugs will be effective against your virus (and its mutations)

In most cases, the genotype is the preferred test. For people who have been on HIV treatment before and may have HIV that is resistant to a number of different drugs, including protease inhibitors, the phenotype test may be done in addition to the genotype.

When Should You Get a Resistance Test?

Resistance testing is recommended for:

  • People who have just become infected with HIV, whether or not they are going to take HIV drugs right away
  • People who have never taken HIV drugs and are starting to receive medical care, whether or not they are going to take HIV drugs right away
  • People who have never been on HIV drugs and are planning to start
  • People who are on HIV drugs and see their viral load go up
  • People who have recently started HIV drugs and their viral load is not coming down enough
  • HIV+ pregnant women

Resistance testing is not usually recommended for:

  • People who have stopped HIV drugs for four weeks or more
  • People with a viral load below 500 copies

Resistance tests are helpful when choosing a drug regimen. The tests are only a guide, however. Other factors, such as past medications, side effects, and adherence must be taken into account as well.


Tropism Test

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Tropism describes the way HIV infects CD4 cells. In order to enter a CD4 cell, HIV attaches itself to receptors on the outside (surface) of the cell. Each CD4 cell has a CD4 receptor and two co-receptors called CCR5 and CXCR4 on its surface. To enter the CD4 cell, HIV must attach to the CD4 receptor and either the CCR5 or the CXCR4 co-receptor.

HIV that uses CCR5 to enter the cell is called CCR5 tropic or R5 virus. HIV that uses CXCR4 is called CXCR4 tropic or X4 virus. Virus that uses both co-receptors is called dual or mixed tropic.

Finding out the tropism of HIV is important because there is a class of HIV drugs that work against CCR5 tropic HIV. This class is called CCR5 antagonists or entry inhibitors. CCR5 antagonists work by attaching to the CCR5 co-receptor on the CD4 cell's surface. This blocks HIV from attaching to the CCR5 co-receptor of that CD4 cell. If HIV cannot attach to the CD4 cell, it cannot enter and infect the cell. CCR5 antagonists only work for people whose HIV is CCR5 tropic.

When Should You Get a Tropism Test?

Tropism testing lets your health care provider know if you have CCR5 tropic virus. It is important that you take this test if you are thinking of starting a CCR5 antagonist. At this point, there is only one CCR5 antagonist available called Selzentry (maraviroc), but there are others in development.


Hypersensitivity Test for Ziagen

The HIV drug Ziagen (abacavir) can cause fatal hypersensitivity reactions in some people. Symptoms of hypersensitivity include rash, fever, fatigue, nausea, vomiting, stomach pain, sore throat, cough, and shortness of breath. People with a certain gene (HLA-B*5701) are at risk for developing this hypersensitivity reaction. Consequently, the US Department of Health and Human Services (DHHS) recommends being tested for HLA-B*5701 before beginning treatment with Ziagen.

When Should You Get a Hypersensitivity Test?

If you and your health care provider are thinking about beginning treatment with Ziagen, it is important for you to get the HLA-B*5701 blood test. If you are found to have the HLA-B*5701 gene, the DHHS recommends that you not take Ziagen.


Bottom Line

Regular lab tests are very important in the management of your health and your HIV. Speak to your health care provider about when you need to take the tests described above and how they can be used to help you make treatment decisions.

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This article was provided by The Well Project. Visit The Well Project's Web site to learn more about their resources and initiatives for women living with HIV. The Well Project shares its content with TheBody.com to ensure all people have access to the highest quality treatment information available. The Well Project receives no advertising revenue from TheBody.com or the advertisers on this site. No advertiser on this site has any editorial input into The Well Project's content.
 
See Also
Understanding Lab Tests I: Complete Blood Count and Blood Chemistry
More on Viral Load Testing

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