HIV Genotypic Resistance Testing

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.

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.

No test preparation is needed.