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About Phenotyping
In simple terms, phenotypic testing is performed by placing samples of a patient's HIV in contact with antiretroviral drugs to observe how the virus reacts. Because the virus is exposed to antiretroviral drugs, phenotypic testing is referred to as being a direct way of measuring drug resistance. Phenotypic testing is quantitative because it is possible to evaluate how much of a drug is needed to stop HIV from growing. This type of detailed information can be valuable to a healthcare provider in making more effective treatment decisions.
Click on the links below to learn more about phenotypic testing.
How phenotypic testing is performed —
Recent advances in technology have made phenotypic testing faster and more useful for frequent use. The following is a detailed description of how phenotypic testing using PhenoSense™ HIV is performed using what is known as recombinant technology.
How a copy of HIV is made:
Step 1. A sample of blood is taken from the patient
Step 2. From the sample, key portions of the genes within the patient's virus are copied. This process is called amplification
Step 3. Once step 2 is complete, the amplified genes are inserted into a laboratory sample of HIV. Genes similar to those copied in step 2 are missing from the laboratory sample, so the laboratory sample of HIV is only a "shell" that cannot grow
Step 4. When step 3 is complete, the laboratory HIV becomes a complete genetic copy of the patient's HIV
Measuring phenotypic drug resistance:
Step 1. The copies of the patient's virus made in the previous steps are exposed to all antiretroviral drugs
Step 2. The ability of the virus to grow in the presence of each antiretroviral drug is evaluated. The virus is exposed to varying strengths, or concentrations, of each antiretroviral drug
Step 3. The ability of the patient's virus to grow in the presence of the drugs is compared to a reference virus that is known to be 100% susceptible to all antiretroviral drugs. The comparison between the patient's virus and the reference virus provides the phenotyping results. These results tell doctors how much of a particular drug is needed to stop the growth of HIV by 50% (compared to how much is needed to stop the reference virus by 50%).
Results — The patient's virus is considered to be resistant to a particular drug if a lot more of the drug is required to stop viral activity than is required to stop activity of the reference virus
What phenotypic test results mean —
The results of phenotypic testing mean that if a person's virus is less susceptible (resistant) to a particular drug, that drug is less likely to work for him or her. However, even if an individual shows resistance to a particular drug, that drug may still be effective, as long the person's level of resistance is not too high. The maximum level of resistance that someone can have before a drug is no longer considered to be effective is called a cutoff value.
Because phenotypic testing measures the amount of drug required to inhibit viral activity, a person's level of resistance can be defined — as the amount of drug increases, so does the level of resistance. With this information, rather than immediately eliminating a drug because resistance is detected, physicians can evaluate a person's level of resistance to determine whether or not the drug is a still a viable treatment option.
How phenotypic results are reported —
After all of the steps involved in performing a phenotypic test are complete, the results must be collected and placed on a report form. Although the way in which the test itself is performed is absolutely critical, if the results are not clearly reported than the information may not be as useful. Click here to view sample phenotypic report form (PhenoSense™ HIV) as an example of how results are reported.
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