HIV Drug Resistance Testing
The ultimate goal of anti-HIV drug therapy is to reduce the amount of HIV in the patient’s blood (viral load) to an ‘undetectable’ level. Anti-HIV drugs work by stopping the virus from reproducing inside the body. These drugs are used in combination and can be effective for a very long time.
What is drug resistance?
When HIV reproduces it can mutate (change) and sometimes these mutations make it possible for the virus to reproduce even in the presence of the drugs. When this happens, the virus is said to have drug resistance and the drugs lose their effect. It is important to note that these mutations cause resistance to some drugs but not others.
Why test for drug resistance?
Drug resistance testing can identify the mutations (changes in the virus) that prevent certain drugs from working. By identifying these mutations, doctors can choose different drugs—drugs that are not affected by the mutations—and return the patient’s viral load to an ‘undetectable’ level.
In some cases, a person can be infected by a virus that already has drug resistance. If the doctor knows this from the beginning, a tailor-made drug combination therapy can be chosen rather than the standard therapy that works well for most people.
How does drug resistance testing work?
The BC Centre for Excellence in HIV/AIDS laboratory extracts genetic material from the HIV in the patient’s blood and uses sequencing technology (similar to DNA fingerprinting of the virus) to identify drug resistance mutations. Lists of specific mutations associated with drug resistance are available in the scientific literature. The laboratory report shows whether the virus has mutations and which anti-HIV drugs may be affected by the mutations.
If the patient has had routine viral load testing done at St. Paul’s Hospital, drug resistance testing can be done using left-over, stored blood samples. In this case the patient does not need to have blood drawn again. The test usually takes about two weeks.
When should patients be tested for drug resistance?
Newly diagnosed patients should be tested even if treatment will not be started right away. Testing will identify patients infected with already-resistant virus and can be used as a baseline. Testing should be done before switching to different drugs to ensure that the new drugs will be effective.
Testing for drug resistance is especially important if a patient is experiencing poor or no response to anti-HIV drugs as indicated by rising viral loads. Test results will help the doctor choose appropriate new drugs for a successful therapy.
Can the likelihood of drug resistance be decreased?
Yes. HIV cannot mutate if it is not reproducing. When anti-HIV drugs are working HIV reproduction is minimal. HIV drugs only work at their best when taken as directed, with no missed pills. This is one key factor in preventing drug resistance.
