Summary

• Genotypic drug resistance testing can help make sure the HIV antiviral medication you are taking is working properly.
• When the virus enters your body, it must make copies of itself to be able to spread.
• In doing this, copying mistakes are often made and mutations (changes to the genetic material in the virus) occur.
• Mutations in the virus can make your medication less effective.
• If drug resistance is found, a new medication can be used instead.

What is HIV genotypic resistance?

HIV genotypic resistance testing looks for mutations in the virus that are known to cause resistance to antiviral medications (medications that treat viruses).

When a virus enters a person’s body, it begins a process to enter cells and make copies of itself. Viruses are composed of a small piece of genetic information (either DNA or RNA) inside a protective shell. Viruses cannot replicate their genetic material on their own — they must infect a host cell to be able to do so.

In the case of HIV, it infects a type of white blood cell called a CD4 T-cell, which is part of the immune system. Inside these cells new HIV copies are made.

When the virus copies itself mistakes often occur. Each mistake changes the virus’s genetic code slightly — these are mutations. Some types of viruses mutate more than others and HIV is a virus that mutates almost every time a new copy of the virus is made.

Antiretroviral drugs (ARVs) work by stopping the virus from copying itself. Some drugs do this by stopping the virus from attaching to or entering CD4 cells, while others block the copying process inside the cell.

If the virus develops mutations, the drug may not be as effective at blocking these processes and will not work. This is when the virus is said to have genotypic resistance.

If your treatment is not working and a known mutation is detected, your treatment will need to be changed.

By continuing with the same treatment, the mutated virus will be able to keep copying itself 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 still being destroyed by the drug.

Not every mutation causes resistance. Some drugs only require the virus to develop one mutation to become resistant, while other drugs will continue to work until the virus develops a number of mutations.

How the test works

HIV resistance testing is used at the start of therapy to help your doctor choose the treatment that will work best for you. It is later used to check that your treatment is working.

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 enough genetic material for reliable testing.

In genotypic resistance testing, the genetic material of the virus is separated from your blood and sequenced – a way to read the genetic material and find mutations. This results in a long string of letters which represent the genetic code 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.

The test is not good at detecting less-common mutations, which affect fewer than 20 per cent of people with HIV. Also, mutations that have not been seen before may be present in rare strains of HIV and these will not be detected by the test.

Having the test

Sample 
Blood.

Preparation
None.

Your result

The test result will report the viral mutations. These are described by a mix 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 treatment.

Questions to ask your doctor

The choice of tests your doctor makes will be based on your medical history and symptoms. It is important that you tell them everything you think might help.

You play a central role in making sure your test results are accurate. Do everything you can to make sure the information you provide is correct and follow instructions closely.

Talk to your doctor about any medications you are taking. Find out if you need to fast or stop any particular foods or supplements. These may affect your results. Ask:

• Why does this test need to be done?
• Do I need to prepare (such as fast or avoid medications) for the sample collection?
• Will an abnormal result mean I need further tests?
• How will the results from this test change the course of my care?
• What will happen next, after the test?

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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