Drug resistance has become a significant obstacle to the long-term success of HIV therapy. In this new era of HIV treatment, it is important that you understand what drug resistance is, how physicians test for it, and what the results of drug resistance testing mean. The following information offers an in-depth discussion of drug resistance testing.

• What is drug resistance testing?
• When should resistance testing be used?
• How expensive is drug resistance testing and is their reimbursement?
• Why is drug resistance testing important?

Drug resistance testing is an important tool in HIV therapy that can help physicians understand more about a person's virus and therapy options. There are two methods of drug resistance testing: phenotypic and genotypic.

Phenotypic testing
Phenotypic testing measures a person's HIV against each of the currently available antiviral drugs. This method detects drug sensitivity by directly exposing the virus to a drug. Once the virus is exposed to a specific drug, it is evaluated to determine how much of the drug is necessary to block viral activity. An individual's virus is resistant to a particular drug when more of the drug is required to block the virus.

Genotypic testing
Genotypic analysis determines the presence of genetic mutations in HIV that may be associated with drug resistance. By using genotypic analysis to evaluate the genetic makeup of someone's virus, the data can be compared to past trends in HIV treatment failure. Included in the knowledge about HIV are preestablished genetic mutation patterns that have been linked to drug resistance. If the genetic mutations in a person's virus match mutations assumed to signal resistance for a certain drug, then his or her virus is presumed to be resistant to that drug.

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Studies have shown that drug resistance testing can help optimize the selection of antiviral medications for initial treatment regimens and when planning new regimens following treatment failure.

Before therapy begins
Because drug-resistant strains of HIV can be passed from one acutely infected person to another, resistance testing can be used to evaluate drug resistance in individuals who have never received therapy. The results can help physicians and patients work together to design a targeted treatment plan that is more likely to be effective for a longer period of time.

Following treatment failure
There are a number of reasons why a person may experience treatment failure. They include: poor adherence, poor drug absorption, and drug resistance.

When treatment failure is due to drug resistance, resistance testing can help determine which drugs or combination of drugs are not working as anticipated. By removing the ineffective drug or drugs from a patient's treatment, a physician can create a new treatment plan that is more likely to suppress the virus.

Consensus Recommendations
Treatment guidelines that include drug resistance testing have been made by the U.S. Department of Health and Human Services and the International AIDS Society - USA Panel. They recommend drug resistance testing for the selection of new antiviral therapy regimens after treatment failure. It is also recommended that drug resistance testing be considered for patients who require, but have not yet received therapy. Resistance testing is recommended for pregnant women to optimize treatment during pregnancy and to select the best therapy to prevent the transmission of HIV to the fetus.

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Drug resistance testing varies in cost between genotypic and phenotypic tests. Genotypic testing can range between $300 to $600 per test and phenotypic testing can range from $700 to $1,000 per test. The difference in cost between genotyping and phenotyping is related to the complexity of the tests, and the laboratory time that is required to perform them.

The important thing to remember when considering the cost of drug resistance testing is the potential expense of not using resistance testing. Because resistance testing helps doctors choose the right drugs and eliminate the drugs that are no longer working, money is not being wasted on ineffective therapy. In the long-term, drug resistance testing can actually result in saving money.1

Because the importance of drug resistance testing in the treatment of HIV has become apparent in the healthcare community, reimbursement has become widely available in the U.S. However, coverage varies between states and payors so you will need to do some research to identify the level of coverage available. For more information about HIV drug resistance testing reimbursement, talk to your healthcare team or call your healthcare insurance provider.

Reference:
1. Weinstein MC, Goldie SJ, Losina E, et al. Use of genotypic testing to guide HIV therapy: clinical impact and cost-effectiveness. Annals of Internal Medicine. 2001;134:440-450.

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When used in combination with viral load and CD4 T cell count analyses, drug resistance testing can give physicians a more complete picture of a patient's health and therapy options. The use of drug resistance testing can help physicians determine which drugs may not work in a patient's therapy, allowing him or her to create a treatment plan that is more likely to suppress the virus for a longer period of time.

In addition to helping physicians select more effective therapies, there may be long-term benefits from using resistance testing to direct treatment decisions. These include avoiding the use of potentially ineffective drugs, and in turn, reducing the health risks associated with medication side effects and the amount of money wasted on purchasing unnecessary medications.

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