PhenoSense™ HIV is the phenotypic HIV drug resistance test most widely used
today. PhenoSense HIV is successful based on the quality of the test —
utilizing state-of-the-art technology for a direct, rapid, and precise measure
of the virus' sensitivity to antiretroviral drugs. Test results allow
healthcare professionals to review the level of susceptibility or resistance
that a person has to each antiretroviral drug in order to design an
individualized treatment plan. PhenoSense HIV drug resistance results are also
provided with a measure of viral fitness, or Replication Capacity (RC).
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 considered 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
practical for frequent use. The following is a detailed description of how
phenotypic testing is performed with PhenoSense HIV, by using 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 sample 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 varying strengths, or concentrations, of all antiretroviral drugs
Step 2. The ability of the virus to grow in the presence of each antiretroviral
drug is evaluated
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%). The
patient's virus is considered to be resistant to a particular drug if much more
of the drug is required to stop viral activity than is required to stop
activity of the reference virus
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What Phenotypic Test Results Mean
If a person's virus is less susceptible (or is 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
required drug increases, so does the level of resistance. With this
information, physicians can evaluate a person's level of resistance to
determine whether a drug is still a viable treatment option rather than
immediately eliminating the drug because resistance is detected.
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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. Clear reporting is
essential for the results to be useful.
Below is a sample PhenoSense HIV report form to provide a better idea of how the
drug resistance and RC information is reported. The report form includes drug
resistance information for all of the approved nucleoside reverse transcriptase
inhibitors (NRTIs), non-nucleoside reverse transcriptase inhibitors (NNRTIs),
and protease inhibitors (PIs).
Click on the image below to enlarge the form.
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