9.5 When to use resistance tests
9.5.1 Recap of previous section
The previous section looked at how resistance is measured and how test results are interpreted.
- There are two main types of resistance tests but genotype tests are used most often.
- Genotype tests report mutations (ie M184V) and phenotype test report fold-changes (ie 4-fold resistance). Both tests should include a detailed interpretation – ie whether each drug is likely to be active (sensitive), partially active (reduced sensitivity) or inactive (resistant).
- Each mutation has a different clinical implication. Some are associated with high level resistance and some with lower resistance.
- Resistance to one drug in a class often means you have resistance to similar drugs in the same class. This is called cross-resistance.
- Resistance can only be tested when viral load is detectable but different labs have different lower viral load cut-offs for the test to work.
- It is important to consider the history of resistance and not just the results of the current resistance test. This includes previous treatment history and previous resistance tests.
- The interpretation of complicated results requires expert advice.
9.5.2 Introduction to section 5
This section looks at when resistance tests should be used based on treatment guidelines.
- UK recommendations are based on BHIVA guidelines (see the monitoring guidelines, adult guidelines, and pregnancy guidelines). The current version when this resource was updated was from 2022.
Other guidelines include:
- European HIV Drug Resistance Guidelines (2011, now out-of-date)
- US treatment guidelines (DHHS)
Most treatment guidelines for Western countries have similar recommendations for resistance testing. The resistance section of the BHIVA adult guidelines is summarised in Appendix 8.
Section 5 also includes access to resistance testing.
- Why are these tests not always given?
- When to advocate for someone who has not been given a test
9.5.3 When to use resistance tests
Genotype resistance tests are recommended when first diagnosed and before most treatment changes (see Table 3).
- When first diagnosed. This is to check for transmitted drug resistance.
- Before starting ART. This is to help with the choice of ART if someone has never had a resistance test or if they have been at risk of reinfection since the first resistance test.
- Before any change of treatment, as long as viral load is detectable.
Table 3: When to use resistance tests
| Stage | Recommendation | Comment |
| Newly diagnosed | Yes, all patients. Both for recent and older infections. | In the UK, 5-15% of newly diagnosed people have at least one mutation. |
| Starting first treatment | Yes, BEFORE STARTING. i) People who have never had a resistance test should have a sample tested before starting treatment. ii) People who may have been reinfected with a new strain of HIV since their first resistance test may be retested before starting treatment. |
i) Test the earliest sample. If this is not available, a current sample should be used. ii) If someone has had other exposures since diagnosis, a resistance test will limit the chance that the first treatment will fail. |
| Changing treatment (viral failure) | Yes. If viral load rebounds on treatment, test for resistance BEFORE CHANGING treatment. Resistance testing can help determine if treatment failure is due to HIV reinfection. | A resistance test BEFORE CHANGING treatment will provide an indication of how much resistance developed while the treatment was failing. Some low level resistance may not be detected. |
| Changing treatment (side effects) | No. A resistance test is not needed if your viral load is undetectable. If this is soon after starting treatment and viral load is still reducing, resistance testing is not needed. | Resistance only develops on failing treatment. Never test when viral load is undetectable. |
| Pregnancy | Women who start treatment during pregnancy should be tested for drug resistance. Resistance testing should be done if viral load is still detectable at delivery. If the women decides to stop treatment after the birth, resistance should be tested six weeks after stopping treatment. | As for starting treatment. Testing if viral load remains detectable is important for future care. Although treatment should be stopped carefully to reduce the risk of resistance, this should be confirmed with a resistance test. |
| Children | Guidelines for resistance testing in children are the same as for adults. In the rare cases (in Western countries) of infants born with HIV, resistance testing should be included with the full panel of other tests. | Resistance develops in children in the same way as it does in adults. Any child on treatment with a detectable viral load is likely to have developed, or be developing resistance. |
| Before using a CCR5 inhibitor (ie maraviroc) | UK and European guidelines recommend using a type of genotype test to check viral tropism. | CCR5 inhibitor only work against CCR5-tropic virus. |
| PEP (Post-Exposure Prophylaxis) | PEP should be started as soon as possible. It should not be delayed waiting for resistance test results. If the HIV positive partner has drug resistance, this will affect the choice of drugs used for PEP. If resistance is discovered later the drugs can be modified. | PEP usually includes an integrase inhibitor as transmitted integrase resistance is uncommon. The urgency with PEP is to first start any combination. |
9.5.4 Which tests to use: genotype or phenotype?
Recommendations for resistance testing refer to using a GENOTYPE test.
This is because genotype tests are cheaper (approximately £200 vs £700) and quicker (1–2 weeks compared to more than two weeks), compared to phenotype tests. Genotype results are also more widely understood.
A phenotype test is generally only recommended if the results from a genotype test are difficult to interpret.
Phenotype tests are only used in people who have very limited treatment choices. This is usually in a case where there is extensive resistance to several different classes of HIV drugs.
Question: Are resistance tests used to see if the type of HIV in different people is in some way linked – in prosecution cases of transmission, for example?
Answer: No. The tests comparing two different viruses are called phylogenetic tests. They are more complicated and expensive tests. It is important to remember that phylogenetic tests can show when people have a similar virus, but not the direction of infection (ie whether one partner infected another).
9.5.5 How to access tests if the guidelines are not followed
Although guidelines are clear, resistance tests are not always provided routinely in all clinics. This is why it is important to know about the current UK guidelines.
If cost is the reason, then complaining to the clinic may help ensure that the test is included as part of current standard of care.
i-Base sometimes get calls from people who were newly diagnosed but didn’t get a resistance test until they asked for it.
- Usually it is sufficient to go back to your doctor and refer to the UK guidelines.
- Some clinics store a sample to test later, before starting treatment. In theory this may be okay, but sometimes samples get lost, or old samples may be difficult to test. In these cases, testing the current sample may not pick up resistance which is present at low levels. There is no real cost-saving from delaying this test.
- If your doctor or clinic will not agree to the test when it is clearly recommended, you can write to the head of your clinic and the head of your health trust. If this is still not provided you may want to register at another clinic to get this test. You can always change back to your local clinic in the future for routine monitoring or treatment.
Please call the i-Base phoneline if you would like further information or support in accessing resistance tests.
9.5.6 Section 5: Learning points
This section has been more practical.
It shows how the technical information in Section 4 is used in real life to decide the choice of ART.
- Treatment guidelines are an important resource, because they state when tests should be used. Most guidelines agree on the use of resistance testing. Guidelines are free to access online.
- Genotype tests are used routinely. Phenotype tests are used when there is more complicated resistance and fewer treatment options. They are used when the genotype results are unclear.
- Guidelines are not always followed, especially for newly diagnosed people.
- Resistance tests and the subsequent results often require active patient or advocacy involvement.
9.5.7 Section 5 evaluation
Please now take a few minutes to evaluate this session online.
This single page includes six short questions.
Your answers are anonymous, but the course is dependent on your feedback, which is appreciated.
Last updated: 1 January 2023.