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Resistance Test

What is "resistance?"

"Resistance" occurs when the medicines you may be taking for your HIV disease can’t stop the virus from making more copies of itself—a process known as replication.

What causes resistance?

Resistance is caused by the way HIV makes new copies of itself.

If you aren’t being treated for your HIV disease, it can make billions of new virus particles every day.

But that replication process isn’t perfect. HIV multiplies so quickly that it makes a lot of genetic mistakes in the new copies. If these imperfect copies of the virus are able to go on to create copies of themselves, they are known as mutations.

Mutations are good for viruses—but they can be bad news for people living with HIV. That’s because HIV medications may not work on mutations. We say that HIV is "drug-resistant" when it can multiply quickly in your body, even though you are taking medications to stop that from happening.

Preventing Resistance

The best way to prevent resistance is to keep HIV from replicating. If it isn’t reproducing, the virus can’t mutate and make new strains of HIV that are drug-resistant.

That’s what antiretroviral drugs do—they keep HIV from reproducing. And that’s why it’s important to take all your HIV medications on time and consistently. When you do that, your medications can do a better job of keeping the virus under control and keep it from mutating into strains that won’t respond to treatment.

Testing Positive AND Resistant

Drug-resistant HIV can be passed from one person to another. That’s one reason why it’s important to have drug resistance testing soon after you test positive for HIV—you need to know as soon as possible if your particular strain of HIV is drug-resistant, so that you and your providers can make the best choices for your care.

Testing for resistance

There are two types of resistance tests: a genotypic assay and a phenotypic assay. These are scientific terms that describe the way each test measures resistance.

  • Genotypic assays look directly at the genetic material of the HIV in your blood and give you information about the HIV drugs your virus is resistant to.
    • Genotypic assays are the most common and widely used resistance tests.
    • They usually take about 1-2 weeks to come back from the lab.
    • They are less expensive than phenotypic assays.
  • Phenotypic assays actually measure how well the virus responds to medications in a controlled environment, such as a laboratory.
    • It typically takes 2-3 weeks to get your results.
    • It is easier for care providers to interpret those results.
    • Phenotypic assays are used if you have multiple HIV mutations and/or multiple forms of treatment have not worked for you.

For more information, see the Department of Veterans Affair’s Just Diagnosed: Resistance Testing.

Fact Sheets & Print Materials

Related Topics on AIDS.gov

Frequently Asked Questions

If my resistance tests show that I am resistant to a lot of HIV medications, does that mean that HIV treatment won’t work for me?

No. Remember - it takes a skilled professional, and often a skilled team of clinicians to interpret these tests. Your providers will use these test results to work with you and make appropriate decisions about your care plan. These tests, while worthwhile, aren’t the final word in your HIV treatment. There are always options.

Last revised: 08/06/2009