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HIV Genotypic Resistance Testing

Also known as: Anti-retroviral drug resistance testing; ARV resistance testing
Formal name: Human Immunodeficiency Virus Genotypic Resistance Testing
Related tests: HIV viral load; CD4 and CD8; HIV antibody; HIV phenotypic resistance testing

At a Glance

Why Get Tested?

If you have been diagnosed with HIV and will be receiving anti-retroviral therapy or if you are receiving anti-retroviral therapy that is not working effectively

When to Get Tested?

When first diagnosed with HIV and/or immediately prior to the start of anti-retroviral therapy or if your HIV viral load levels rise steadily even though you are receiving anti-retroviral therapy

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?

None

The Test Sample

What is being tested?

Resistance is a term used to describe the condition in which a microorganism is able to grow and/or multiply in the presence of one or more antimicrobial drugs. Sometimes when antimicrobials are used to treat an infection, a mutation or change occurs in one of the genes in the microorganism causing the infection. The mutation may allow that microorganism to develop resistance — to grow or multiply despite the presence of the antimicrobial drug. Microorganisms without the mutation are killed, but those that have the mutation quickly multiply and begin to predominate. The antimicrobial is then no longer effective in treating the infection. In HIV infections, mutations in the viral genes can occur during treatment that allow the virus to replicate and develop resistance to anti-retroviral drugs (ARVs). A person can also be infected with a strain of HIV that is already resistant to certain drugs. In such cases, resistance testing can provide information to the doctor even before therapy is initiated.

There are a few different methods that can be used to determine if HIV is resistant to ARV drugs. If you are infected with HIV and are currently on active ARV therapy but your viral load continues to increase, then you may undergo either genotypic or phenotypic resistance testing. In genotypic resistance testing, the genetic code of the HIV you have been infected with is checked in detail to see if there are any genetic mutations or changes that are known to cause drug resistance. HIV mutates almost every time a new copy is made, but not every mutation causes resistance. At any given time there is always more than one genetic form of the virus (drug-resistant and drug-sensitive) that can multiply simultaneously in a patient. In the presence of a drug, however, the resistant population will outgrow the drug-sensitive HIV. This is called “selective pressure” because the drug “selects” the genetic forms of the virus that are resistant to it.

To avoid the development of ARV resistance, it is usually recommended that you be treated with a combination of drugs that are from two different classes of ARV. This is known as highly active retroviral therapy or HAART. There are many different ARVs approved by the U.S. Food and Drug Administration that fall into different categories. For more on this, visit the National Institute of Allergy and Infectious Diseases webpage on Treatment of HIV Infections.

How is the sample collected for testing?

The test is performed on a sample of blood drawn from a needle placed in your arm.  Genotypic resistance testing works best on blood samples with a viral load of at least 1,000 copies per milliliter of blood. If your viral load is very low, the test probably won’t work and it may not detect resistance to all ARVs.  However, it is recommended that testing be considered for those whose viral load is between 500 and 1,000 copies/mL.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

The Test

How is it used?

If you are infected with HIV, genotypic resistance testing is used to detect the presence of one or more mutations in the virus causing your infection that enable it to be resistant to certain anti-retroviral drugs (ARVs). The mutations will specify the drugs to which the virus is resistant. Your doctor will then be able to select a treatment that will be effective. Genotypic testing helps your doctor make better treatment decisions when considering which drugs to use before beginning therapy or when the ARV you are taking is not working effectively.

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When is it ordered?

HIV genotypic resistance testing may be ordered when you are first diagnosed to determine whether you are infected with a strain of HIV that is known to be resistant to certain ARV regardless of whether you will begin treatment right away. It may also be ordered immediately before the start of therapy if a drug-resistant virus is suspected, so that immediate, appropriate therapy is possible. Testing may also be ordered when viral load values (a measure of how much HIV is in your body) rise steadily during therapy, indicating treatment failure and the possibility of resistance. Generally, a treatment is considered a failure if your viral load rises more than three-fold in consecutive tests. If drug resistance is found, a new treatment regimen may be chosen.

Genotypic resistance testing should be performed on all HIV-positive pregnant women prior to starting therapy and for those women who become pregnant while on ARV and who have detectable viral loads.

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What does the test result mean?

Results of genotypic resistance testing must be interpreted with caution. Often, a specialist is consulted when considering results of the test and selecting appropriate ARV. The test results identify the viral mutations. These are described by a combination of letters and numbers (for example, K103N) where the letters refer to the amino acid associated with the gene and the number refers to the position of the mutation in the genome. Based on the test result, your doctor will determine whether a given mutation is one known to cause drug resistance. Not all mutations cause drug resistance. Some mutations are very common, and resistance is known to certain drugs and/or drug combinations. This is important information for your doctor to help predict which treatment regimen will work best for you. The International AIDS Society—USA maintains a list of primary and secondary mutations that are most commonly associated with resistance to various ARVs and ARV categories. The list is intended to help interpret the results of resistance testing and in selecting appropriate ARV therapy.

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Is there anything else I should know?

Genotypic resistance testing is not recommended if your viral load is less than 500 copies/mL. There is insufficient amount of HIV RNA for testing to be reliable. Testing works best if the viral load is greater than 1,000 copies/mL but is recommended even if the level is between 500 and 1,000 copies/mL.

HIV genotypic resistance is a qualitative test and detects mutations associated with drug resistance. It does not give you the level of resistance associated with the drug. Phenotypic resistance testing is a better indicator of the level of HIV drug resistance. HIV genotypic resistance testing cannot detect unknown resistance mutations that may be present in various strains of HIV. The test may not detect a drug-resistant strain of HIV if the resistant virus makes up less than 10-20% of total virus that is present circulating in the blood.

Testing is not recommended after discontinuing a drug. This is because the percentage of drug-resistant strain may decrease considerably in the absence of the drug. When “selective pressure” from a drug is absent, the drug resistant strain may drop enough to the point where testing would not be able to detect it.

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Common Questions

1.  How long will it take to get the results of my test?

Depending on where you have your test done, your sample may be sent to a reference laboratory. Results may take one to two weeks.

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2.  What is HIV phenotypic resistance testing?

HIV phenotypic resistance testing is the same as genotypic testing in that it tests the strain of HIV causing your infection for resistance to specific ARV but uses a different method. In the lab, a sample of the virus grown in culture is mixed with different concentrations of one or more drugs. If the virus is able to replicate in the presence of the drug, as compared to a known strain of virus (a reference virus), it is said to be resistant to that drug. This type of testing usually takes longer than genotypic resistance testing.

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Ask a Laboratory Scientist

This form enables you to ask specific questions about your tests. Your questions will be answered by a laboratory scientist as part of a voluntary service provided by one of our partners, American Society for Clinical Laboratory Science. If your questions are not related to your lab tests, please submit them via our Contact Us form. Thank you.

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Article Sources

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Sources Used in Current Review

(Revised May 17, 2008) AIDS InfoNet. HIV resistance testing. Available online at http://www.aidsinfonet.org/fact_sheets/view/126 through http://www.aidsinfonet.org. Accessed February 2009.

Clarke, W. and Dufour, D. R., Editors (2006). Contemporary Practice in Clinical Chemistry, AACC Press, Washington, DC. P. 490.

(Oct. 30, 2008) Spacek lL. Johns Hopkins HIV guide, Resistance Testing: Genotype. Available online at http://www.hopkins-hivguide.org/management/laboratory_testing/resistance_testing__genotype.html through http://www.hopkins-hivguide.org. Accessed February 2009.

(Updated February 19, 2009) National Institute for Allergy and Infectious Diseases. Treatment of HIV Infections. Available online at http://www3.niaid.nih.gov/NR/exeres/A01539FA-7571-48C4-8A76-C526D5F7EFE9.htm through http://www3.niaid.nih.gov. Accessed February 2009.

(Updated November 3, 2008) AIDSinfo. Guidelines for the Use of Antiretroviral Agents in HIV-1-Infected Adults and Adolescents. Pp. 9-12. PDF available for download at http://aidsinfo.nih.gov/contentfiles/AdultandAdolescentGL.pdf through http://aidsinfo.nih.gov. Accessed February 2009.

(Update December 2008) International AIDS Society-USA. Update of the Drug Resistance Mutations in HIV-1: December 2008. Available online at http://www.iasusa.org/resistance_mutations/ through http://www.iasusa.org. Accessed February 2009.

Sources Used in Previous Reviews

Thomas, Clayton L., Editor (1997). Taber’s Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].

Pagana, Kathleen D. & Pagana, Timothy J. (2001). Mosby’s Diagnostic and Laboratory Test Reference 5th Edition: Mosby, Inc., Saint Louis, MO.

Janice K. Pinson MT, MBA. Molecular Business Strategies, Birmingham, MI.