Drug Resistance Tests
Currently there are 2 types of
drug resistance
tests, genotypic and phenotypic. Both can help identify which HIV medications the virus will not respond to.
You may need to take a
drug resistance
test if you have just been diagnosed with HIV, your viral load
suddenly rises, you have to change your medication because of side effects, or you are pregnant or breast-feeding.
Genotypic Test
Phenotypic Test
Virtual Phenotypic Test
Genotypic Test
With a genotypic test, a sample of your blood is examined to see if there are any HIV
mutations present.
Once the
mutations
have been identified, they are compared with a long list of
mutations
known to cause resistance to specific
antiretroviral (ARV) medications.
These results ultimately will help a doctor determine to which
ARV medications
your HIV would be least resistant.
Compared with another type of resistance test known as the phenotypic test, the genotypic test is not as expensive, it is less complicated and the results come back more quickly. The only issue with the genotypic test is that because there are so many factors involved in HIV
resistance,
it is not always easy to predict which drugs your virus would be resistant or susceptible to.
Back to Top
Phenotypic Test
Phenotypic tests look for drug resistance
in a different way from genotypic tests. Instead of finding specific
mutations,
these tests measure the ability of the HIV infection to reproduce in the presence of ARV medications.
To do this, blood samples containing your HIV are divided into many test tubes. The virus in each of the tubes will be exposed to varying strengths of each
ARV medication. Laboratory technicians can then calculate how much of each ARV medication would be needed to keep your HIV from reproducing.
The advantage of the phenotypic test is that it tells you exactly which drugs your HIV will be resistant to. Also, you do not need to have an understanding of genetics and HIV
mutations.
Back to Top
Virtual Phenotypic Test
The virtual phenotypic test is a combination of genotypic and phenotypic testing. First, a genotypic test is done to see if there are any mutations.
Phenotype resistance testing involves obtaining a genotype from a patient's viral strain and then analyzing this mutation data with statistical models that correlate genotype to phenotype using large databases containing genotype and phenotype information from individual viral isolates. These models assign resistance weight factors to all mutations present in any given sequence, the weight factors are then added and converted into a Fold Change value which is interpreted using cut-offs that reflect drug activity. This test, the vircoTYPE HIV-1 (www.vircolab.com), combines the benefits from genotyping and phenotyping. Please refer to local guidelines for recommendations on resistance testing.
The virtual phenotype is faster and less expensive than a phenotypic test, combining both quantitative (phenotypic) and qualitative (genotypic) data for an accurate picture of where each ARV drug falls on the resistance continuum.
Back to Top
