Summary

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. 

Why get tested?

Genotypic resistance testing looks for mutations in the virus that are known to cause resistance to specific antiviral medications. HIV mutates almost every time a new copy of the virus is made, but not every mutation causes resistance. 

If your treatment isn’t working and a known mutation is detected your medication will need to be changed. By continuing with the same medication, the mutated virus will be able to keep multiplying because the medication cannot stop it. It will become the most common form of the virus in the body – all the other forms of the virus are being destroyed by the drug. 

For certain drugs, single mutations of a gene will increase resistance to high levels. For other drugs, a combination of mutations is required for resistance to form. 

The test is not good at detecting less-common mutations, which affect less than 20 per cent of the virus population. Also, unknown resistance mutations may be present in rare strains of HIV.

The test is also used at the start of therapy to help your doctor choose the most appropriate and effective combination of antiviral medication.

How the test works
Your doctor will first order a test that measures the viral load. This is a measure of how many HIV particles are in your body. If the viral load is high this suggests your antiviral medication is not working.

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 not sufficient genetic material for reliable testing.

In genotypic resistance testing the genetic material of the virus is isolated from your blood sample 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. 

Having the test

Sample 
Blood 

Preparation
None

Your result

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 adjust your antiviral medications to find the most effective combination. 

Any more to know?

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.

The test involves several time-consuming processes in the lab and results may take several weeks to become available.

More information

Pathology and diagnostic imaging reports can be added to your My Health Record. You and your healthcare provider can now access your results whenever and wherever needed.

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