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Resistance Basics |
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Click on each of the questions below to go to a page answering that question.
What Is HIV?
HIV (human immunodeficiency virus) infects CD4 cells in the body, resulting in the death of these cells. CD4 cells, also known as helper T cells, are important cells in the immune system that help fight off infection and disease.
From the first day of infection, HIV makes copies of itself (replicates) continuously, which means that the amount of HIV in a person's body can increase very rapidly. When the amount of active HIV in the blood or tissues (viral load) overwhelms the immune system, a person is unable to ward off disease. This increases the danger of developing life-threatening illnesses and progressing to acquired immunodeficiency syndrome (AIDS).
AIDS is the severe progression of HIV infection. An AIDS diagnosis is given when the number of CD4 cells in an individual's blood, or CD4 cell count, is at or below 200 and/or when the individual develops infections or cancers that typically occur in people with a severely damaged immune system.
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How Is HIV Treated?
The goals of HIV therapy are to slow the progression of HIV, restore the function of the immune system, and improve a person’s quality of life. To achieve these goals, a healthcare provider may prescribe HIV therapy, including a combination of antiretroviral drugs.
Currently, four classes of drugs are used in HIV therapy:
- Nucleoside reverse transcriptase inhibitors (NRTIs)
- Non-nucleoside reverse transcriptase inhibitors (NNRTIs)
- Protease inhibitors (PIs)
- Entry inhibitors (EIs)
These antiretroviral drugs interrupt the HIV replication process and help preserve the health of the immune system. Therapy with a combination of three or more antiretroviral drugs is usually required for effective treatment of HIV infection.
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What Is HIV Drug Resistance?
HIV drug resistance means that the virus can adapt, grow, and multiply in the presence of drugs. HIV is considered to be drug resistant when a drug or class of drugs is no longer effective against it.
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What Causes Drug Resistance?
HIV replicates very rapidly and makes many mistakes (mutations) in the process. However, HIV doesn't have the ability to correct these mistakes. This results in mutant viruses that can be resistant to one or more of the drugs used in HIV therapy. These mutant viruses continue to make copies of themselves, further reducing the effectiveness of an individual's HIV therapy.
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How Common Is Drug Resistance?
According to recent data, in three out of four people currently taking HIV drugs, treatment failure is linked to drug resistance. Additionally, one in four newly infected individuals is already resistant to at least one HIV drug.
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Why Is Drug Resistance Testing Important?
When used in combination with treatment history, viral load, and CD4 cell count information, drug resistance testing gives healthcare providers a more complete picture of an individual's therapy options. This allows providers to develop a treatment plan that is more likely to be effective.
In addition, resistance testing may help avoid unnecessary drug side effects and medical costs associated with taking drugs that are not likely to work.
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How Is Drug Resistance Tested?
A blood sample is taken and sent to a laboratory where one or both types of resistance testing — phenotypic and genotypic — are performed.
Phenotypic testing is performed by testing a sample of a person's HIV against all of the available antiretroviral drugs. By directly measuring the ability of HIV to grow in the presence of these drugs, the laboratory can determine which drugs will work and which are no longer good options. The activity of a person's HIV in the presence of the antiretroviral drugs is compared to the activity of a control strain of HIV that is known to be susceptible to all drugs. This comparison determines how well a drug is likely to work.
Genotypic testing is performed by identifying genetic mutations, or changes in genes, in an individual's HIV that are known to be associated with drug-resistant HIV. Once the mutations have been identified, a computer is usually used to interpret the results for the healthcare provider.
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Can Phenotypic and Genotypic Tests Be Used Together?
Yes. PhenoSense GT™ is a test from Monogram that combines both types of tests to make up one convenient evaluation of drug resistance that is measured from the same blood sample and is reported on the same report form. The PhenoSense GT report displays phenotypic and genotypic results side by side so that healthcare providers can select the best possible therapy for a person living with HIV. PhenoSense GT is direct, uses proven technology, and offers consistency because both phenotypic and genotypic results come from the same blood sample.
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What Do Drug Resistance Test Results Look Like?
The report forms used for Monogram's resistance tests include genotypic and/or phenotypic drug resistance information for all of the approved nucleoside reverse transcriptase inhibitors (NRTIs), non-nucleoside reverse transcriptase inhibitors (NNRTIs), and protease inhibitors (PIs). To download PDFs of sample report forms for each test, click on the links below:
GeneSeq™ HIV
PhenoSense™ HIV
PhenoSense GT™
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When Should Drug Resistance Testing Be Used?
Before therapy begins
Because drug-resistant strains of HIV can be passed from one person to another, resistance testing can be used to evaluate drug resistance in recently infected or newly diagnosed people. The results can help a healthcare provider work with an individual to design a targeted treatment plan that is more likely to be effective for a longer period of time. By using information about how resistance develops when certain drugs are used, healthcare providers can design combinations of drugs that will preserve more treatment options if therapy failure occurs later on down the road.
Following treatment failure
When a person no longer benefits from his or her HIV therapy (treatment failure) and viral load is increasing, drug resistance testing can help determine which drug or combination of drugs is no longer effective. A treatment plan can then be developed that is more likely to slow HIV replication.
Throughout therapy
Drug resistance testing can also be used during the course of an individual's therapy. Periodic testing when HIV is detected in plasma can help gauge therapy effectiveness and drug resistance, so that treatments can be altered as needed.
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How Can HIV-Positive People Prevent Drug Resistance?
In addition to working with their healthcare providers and using drug resistance tests as appropriate, people living with HIV can fight drug resistance by:
- Taking HIV drugs on time, every time. If people under HIV treatment skip their medications, stop taking them or don't stick to their schedules, it becomes easier for the virus to develop resistance.
- Not sharing needles or having sex without a condom with someone else who has HIV. This way, HIV-positive people avoid exposure to additional, drug-resistant strains of the virus.
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How Can I Learn More About Drug Resistance Testing?
For more detailed information about drug resistance testing and its role in HIV therapy, talk to your doctor. You can also download the No-Nonsense Guide to HIV Drug Resistance Testing below.
Click here to download a PDF of the No-Nonsense Guide to HIV Drug Resistance Testing in English
Click here to order the No-Nonsense Guide to HIV Drug Resistance Testing
Click here to download a PDF of the No-Nonsense Guide to HIV Drug Resistance Testing in Spanish
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| Is an HIV drug resistance test right for you? If so, which kind? Click here |
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Find out what HIV experts say about resistance testing.
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| How has testing affected your HIV therapy? Click here |
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