Genotypic drug resistance testing can help your doctor make sure the Human Immunodeficiency Virus (HIV) antiviral treatment you are taking is working effectively. If drug resistance is found, a new treatment can be used instead. 
 

What is being tested?

he genetic material of the Human immunodeficiency virus (HIV) is isolated from the blood sample. The genetic material is then sequenced – a form of decoding. This results in a long string of letters signifying the genetic code of particular parts of the virus. The sequence from your virus is compared to a computer database to see if any genetic mutations that are known to cause drug resistance are present.

In genotypic resistance testing, the genetic code of the particular strain of HIV a patient has is checked to see if there any genetic mutations that are known to cause drug resistance. HIV is said to be resistant to an antiviral medication if it keeps multiplying while a person is taking the drug. Changes (mutations) in the virus cause resistance. HIV mutates almost every time a new copy of the virus is made, but not every mutation causes resistance. Antiviral drugs control most types of HIV; however, a strain of virus containing a mutation that is resistant to a drug will multiply and become the most common form of the virus in the body (as all the other forms are destroyed by the drug). For certain drugs, single mutations of a gene increase resistance to high levels. For other drugs, a combination of mutations is required for resistance to form.

How is it used?

Genotypic resistance testing helps doctors make better treatment decisions for their patients when existing anti-viral therapies are not working effectively. If drug resistance is found, a new treatment regimen may be chosen.

When is it requested?

Genotypic resistance testing is requested when viral load values (a measure of how many HIV particles are in your body) rise steadily during therapy, indicating treatment failure and the possibility of resistance. The test is also used at the start of therapy to help your doctor choose the most appropriate and effective combination of antiviral medication.

What does the result mean?

The test result identifies the viral mutations. These are described by a combination of letters and numbers, for example K103N. Based on the test result, your doctor will identify whether a given mutation is one known to cause drug resistance. Not all mutations cause drug resistance. Your doctor can then adjust your antiviral medications to find the most effective combination.

Is there anything else I should know?

The test is not good at detecting 'minority' mutations, which affect less than 20% of the virus population. Unknown resistance mutations may be present in rare strains of HIV.

Genotypic resistance testing works best on blood samples with a viral load of at least 1,000 copies per millilitre of blood. If your viral load is very low, the test probably won't work as there is sufficient genetic material for reliable testing.

This test involves several time consuming processes. The test results may take several week to become available.

Common questions

• How much does it cost?

Genotypic resistance testing is now used routinely by HIV specialist doctors in Australia. However the high cost of the test prevents it being widely available in some places.