Resistance Testing

The most common test to measure possible drug resistance is known as ‘Genotyping’. The purpose of this test is to detect the presence of known virus mutations associated with drug resistance.

This test is called a ‘genotypic resistance assay’ or ‘GRA’. It is used to compare the genetic code of the sample of HIV virus being tested against a ‘wildtype’ (the most common form of HIV virus). This test can only be performed if you have a viral load over about 2000 copies per millilitre of blood.

Knowing which treatments you are potentially resistant to and which treatments are effective against your virus is useful in determining your optimal treatment strategy. The current treatment guidelines suggest that this test should be performed:

• prior to commencing treatments;
• to assist in correctly selecting treatments when considering changing treatments;
  
• if there is indication of viral load change during treatment; and, less often,
  
• within 4 weeks after discontinuing or stopping treatments.
  

Another test used to measure resistance is known as ‘phenotyping’. This test is called a ‘phenotypic assay’. It measures the virus’s ability to grow in the presence of different combinations of antiretroviral treatments. This test provides a direct and quantitative measure of the likelihood of resistance developing for individual treatments and can also be used to determine the optimal dosing of treatments.

The third approach to resistance testing is the ‘virtual phenotype’. This test is really a genotype test that is interpreted with the aid of a large database of samples of known genotype and phenotype data. One drawback of this particular form of resistance testing is that the results are dependant on the number of known matches, but its main strength is that for people not on new drugs, as it is a simpler method of determining the likelihood of developing resistance.

Phenotyping is still relatively expensive compared to genotyping and virtual phenotyping, and is currently not available in Australia. As all of these tests are currently not covered under Medicare, the availability and cost of these tests varies. Your doctor or treatments officer will be able to provide more information as to the cost and availability in your area and what these tests may mean for you.

Abacavir Hypersensitivity

This test is rapidly becoming widespread and is a genetic test used to determine the likelihood of a possibly fatal side effect of Abacavir (an HIV antiviral drug) known as Abacavir hypersensitivity reaction. Wherever possible, it should be performed by your doctor prior to commencing Abacavir.

Therapeutic Drug Monitoring (TDM)

Therapeutic drug monitoring (TDM) is used to help individualize anti-HIV therapy by measuring the amount of drug in an individual’s blood (plasma) or cerebral (spinal) fluid. This is important because different people absorb, process, and eliminate drugs at different rates, and blood and cerebral fluid levels may vary considerably among individuals taking the same doses of the same medications. Ideally, the lowest plasma drug concentration between doses (the trough level, or Cmin) should still be high enough to inhibit HIV, but the highest concentration (the peak level, or Cmax) should not cause intolerable side effects.

Some, but not all, studies have shown that using TDM to guide treatment decisions increases the chance of successful viral suppression and can assist in minimising side effects; however, drug level monitoring is not appropriate for all anti-HIV drugs.