What is resistance?

Drug resistance occurs when the structure of a virus makes tiny changes that stop the treatment from working. These changes are called mutations.

• You cannot develop resistance if you are not taking treatment.
• You can be infected with a strain of HIV that is already resistant to some or all HIV drugs.

About 7% of new infections in the UK are resistant to one or more drugs.

This is why in the UK everyone should have a resistance test when they are diagnosed and before starting treatment.

You may need to ask for this test.

How does resistance occur?

Mutations that lead to drug resistance are generally only produced if you continue taking a treatment when you have a detectable viral load.

If your viral load is still above 500 copies/mL after 2-3 months, or above 50 after 6 months, you may have developed resistance and may need to change drugs.

Your doctor should look for why the results are not as good as they could be.

Your doctor will also want to discuss how you are managing adherence and side effects. This may include tests for resistance and possibly drug levels.

Resistance can develop even at viral load levels between 50 and 500 copies/mL.

You should have a viral load test four weeks after starting or changing treatment. This should then be checked every 3–4 months when on treatment.

Get the results when they are ready, usually within two weeks. Don’t just wait until your next routine visit.

Some clinics let you get your blood tested 2-3 weeks before you see your doctor. Then you will have the results back for the appointment.

What happens if my viral load rebounds?

If your viral load has increased, you should get a second test when you get these results, or soon after.

Often slight increases are due to errors in the test. You can also have small increases that go back down again that are called ‘blips’ or ‘spikes’.

The second test will check what is happening. If the combination is failing then you reduce the risk of further resistance by checking this straight away.

You will get a better response to a second treatment if you change when viral load levels are still low.

What is cross-resistance?

Cross-resistance is when resistance to one drug causes resistance to other similar drugs, even if you have never taken them before.

This is particularly true of drugs in the same class.

So if you develop resistance to one NNRTI such as efavirenz then you may also have resistance to other NNRTIs such as nevirapine.