Drug resistance

What is resistance?

Resistance to anti-HIV drugs occurs when part of the structure of the virus makes a minor change which we call mutation. The mutated virus avoids the effect of the drug, which means that the drug does not act in the best way, or that it has no effect on the virus.

Also, you can be infected from the start with a virus resistant to some or all drugs.

Tests on the resistance of the virus should be performed when HIV status is diagnosed and before beginning of the therapy. In the Republic of Serbia there is still no possibility to perform these tests.

How does the resistance occur?

Mutations that cause resistance occur when the viral load is still high enough (over 50 copies).
If the viral load remains above 500 copies per milliliter of blood after two or three months, or more than 50 copies after six months, you should consider changing therapy.

Resistance can occur when the amount of virus per milliliter of blood is between 50 and 500 copies.
Your doctor should find the reasons why the results are not as good as they could be. Talk about how committed you are while taking regular therapy and how you tolerate side effects. You should do a test on the resistance, as well.

It is recommended to test a viral load in blood after four weeks after initiating or changing therapy, and continue every three months.

Get your results as soon as they are completed (usually two weeks). Do not wait to find out until the next time you visit your doctor. It would be best to do a blood test 2-3 weeks before your next visit to the doctor - in this case, the results would be ready for your appointment.

If the viral load is increased, do the new test on the same day, in order to confirm the result. Sometimes a small increase is a mistake in a test result.

Re-test will find out what is happening. If you use the therapy successfuly, this will reduce the risk of further progression of resistance.

The success of therapies will be greater if you start with it when your viral load is still low.

What is cross-resistance?

Some drugs cause cross-resistance, which means if resistance occurs to one drug, it will occur to another drug similar to the first one, even the one you have not previously been treated with. This is especially true for drugs that belong to the same group.

The level of cross-resistance also varies, and benefit from the use of another drug may be noticeable, but rarely strong enough or durable.

How to avoid a resistance?

Avoiding resistance is one of the most important conditions for the use of combination therapies. You must make sure that the combination you have chosen is strong enough to minimize the risk of developing resistance to any of the medicines you are taking.

The reason for using at least three or four drugs is that the virus continues its development and mutation, even when his presence in the blood is between 50 and 500 copies.

The best way to prevent development of resistance is to maintain the amount of viral copies in the blood below the 50.

The number of viral copies in the blood during treatment:

less than 50 copies / ml of blood
Very small amount of the virus multiplies in the blood, so it is unlikely to develop resistance to combination you are using. Carefully and regularly receiving treatment will allow you to use it for many years.

between 50 and 500 copies / ml of blood
This is not small enough viral load to prevent development of resistance. At a time when resistance becomes widespread enough, the drugs will stop to work and the viral load will start to grow.

over 500 copies / ml of blood
If you continue to use the same combination while the number in your blood is still measurable (and it would not start to decline), there is a high risk to develop a resistance, which means you will be able to use your combination of drugs not for much longer.