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About Genotyping
Genotypic testing looks for the presence of genetic changes, or mutations, in HIV to predict resistance to antiretroviral drugs. If the mutations found in a person's virus match the mutations that are known to cause resistance to a drug, then his or her virus is presumed to be resistant to that drug. The critical aspect of genotyping though is not identifying mutations but rather in the interpretation of the results -- in other words, making sense of all the different mutations and how they interact with each other to affect drug susceptibility.
Click on the links below to learn more about genotypic testing.
How genotypic testing is performed —
There are two main types of genotypic resistance testing: sequencing assays and point mutation assays. Sequencing assays scan the complete sequence of gene, whereas point mutation assays look for mutations at specific locations, or codons, in the gene sequence. Though these assays differ in how the genetic structure is analyzed, they both start by using polymerase chain reaction (PCR) technology to amplify (copy) the viral RNA or DNA present in a patient's blood sample. PCR technology allows researchers to amplify specific genetic sequences in a person's HIV to create a sufficient amount for genotypic testing. The following example of how genotypic testing is performed is based on GeneSeq™ HIV.
How the test is performed
Step 1. Sample is taken
Step 2. Copies of the virus are made (using PCR technology, the genetic material of a person's HIV is copied numerous times)
Step 3. Genes within the virus [most commonly the reverse transcriptase and protease genes] are evaluated for mutations
Step 4. Mutations found in the virus are compared to known resistance mutations
Results: If the type and pattern of mutations present in a person's virus match pre-established mutations for a particular drug, the virus has probably developed resistance to that drug.
What genotypic test results mean —
If genotypic testing reveals drug resistance mutations in a person's HIV, certain antiretroviral drugs may be less likely to work. The specific type and placement of the mutations determines which drugs the virus may be resistant to. For example, if the "M184V" mutation is discovered in a person's HIV, the virus is probably resistant to 3TC. Once a healthcare provider understands which drugs are less likely to work s/he can develop a treatment plan that avoids the use of those drugs. This results in a therapy regimen that is more likely to be effective for a longer period of time.
How genotypic results are reported —
Once the mutations present in a person's virus are identified, they are organized and placed on a report form. Mutations are usually categorized by the drug category they pertain to — NRTI, NNRTI, or PI. Once the mutations are organized, they must be reviewed and compared to the most recent guidelines that provide an explanation of what a given mutation, or collection of mutations means; this step can be subjective. Click here to view sample genotypic report form (GeneSeq™ HIV) as an example of how results are reported.
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