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HIV Resistance

Understanding Drug Resistance Testing


Updated September 16, 2007

There are more HIV treatment options than ever before. Choosing an effective HIV regimen is becoming more complicated everyday. But because of these numerous medication choices, people are living longer. Obviously this is a great thing. However, as people live longer with HIV, the chance their virus will mutate and become resistant increases. HIV resistance is a common problem that impacts anyone taking HIV medications.

So what happens after HIV resistance has occurred? After becoming resistant to several drugs, how does the doctor decide which medications to try next? How does the doctor assemble a regimen he feels gives the patient the best shot at an effective regimen? Simply put, doctors rely on advances in technology to help them decide. More specifically, drug resistance testing is used to give the physician an idea of which medicines may work and which should be avoided. Let's take a closer look at HIV resistance.

What is HIV Resistance?

Simply put, HIV resistance means that mutations or changes to the genetic structure of HIV has occurred, making the virus resistant to HIV medication regimens. In other words, the genetic changes that have occurred allows the HIV virus to reproduce despite the presence of HIV medications.

Understanding HIV Replication

Why Does HIV Resistance Occur?

As cells reproduce, they make exact copies of themselves, growing in number with each replication cycle. Ideally, each copy is exactly like the last. But sometimes, small errors in one cell will be passed on to the next during replication. After time, cells that contain these small errors become larger in number. These small changes in the cell's genetic make-up are called "mutations". Mutations are common in HIV because of the rapid pace at which it replicates and it's lack of mechanisms to correct errors. Mutations cause the drugs to be resistant to HIV medications. In other words, HIV resistance has occurred.

The Step-By-Step Process of HIV Replication

What Causes Mutations?

Usually, mutations occur within a cell when exposed to certain conditions or factors. Examples of these conditions include:
  • environmental stress
  • exposure to toxins
  • repeated exposure to certain medications
While HIV medications prevent most HIV replication, some of the virus mutates, allowing a resistant strain of the virus to grow in number. As this number of these mutated HIV copies grow, the HIV medications become less and less effective. After time, mutations make choosing the right combination of drugs very difficult.

Are There Tests That Detect HIV Resistance?

There are simple blood tests that can detect HIV resistance and help doctors identify which HIV medications will work best against the mutated virus. Two types of resistance testing are now available, genotypic testing and phenotypic testing. Each can help determine if resistant virus is present and if so which drugs the virus is resistant to. By knowing this information, doctors can make a more informed decision when constructing HIV medication combinations. But what is genotypic and phenotypic testing?
  • Genotypic Testing
    Genotypic testing looks at the HIV present in a person's blood and examines it to see what mutations if any exist. Certain drugs are known to cause certain genetic mutations. From knowing if a certain genetic mutation is present, doctors can deduce which drugs or class of drugs a particular virus may be resistant to. For example, if a person's virus contains the genetic mutation known to be caused by Epivir, then doctors can be fairly certain that Epivir will not be effective against this particular strain of HIV. Genotypic testing is relatively fast, inexpensive test that is available to most patients.

  • Phenotypic Testing
    Phenotypic testing differs from genotypic testing in that it takes virus and exposes it to different concentrations of HIV medications to determine which drugs are effective. This method is used early in drug development long before they are given to humans. Phenotypic testing is a slow, expensive that few patients have access to.

Blood Test Fact Sheets

How Do These Tests Impact HIV Care?

As mentioned earlier in this feature, resistance testing assists doctors in constructing therapies for those patients exposed to multiple drug combinations. However, it's becoming more and more common that newly diagnosed people are found to be infected with HIV resistant virus. In other words, people are being infected with HIV that is already resistant to some medications. Obviously, this makes choosing their therapy more difficult. Combinations that would normally work in the newly diagnosed patient is no longer effective due to the resistant virus they were infected with. As we all know, how we treat people in those early stages of infection can greatly affect their course and prognosis. With resistance testing, early therapies can be constructed according to which drugs will be effective against their already mutated strain of HIV.

What Will it Cost Me?

Currently, most insurance companies, private and public, pay for genotype testing. Phenotype testing may not be covered by some insurances. As these tests become more widely used, coverage may expand. In fact, resistance testing has been recommended by experts as an essential part of HIV care.

Insurance benefits Fact Sheets

It's obvious that resistance testing is a very important part of your HIV care. Talk to your doctor to see if resistance testing could benefit you.

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