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understanding resistance testing
Feb 24, 2004

I recently had my first resistance test (after 24 yrs of being HIV+), it was ordered by my Dr. because after 2.5 yrs of undetectable viral load I had some slight increase, (the first test in June less than 100 and in August less than 75). When my October blood was taken my Dr decided to do a resistance test; when the viral load came back it was back down to undetectable and he did not expect the resistance test to yield a result since there was so little virus (undetectable). A week later we recived a test result showing resistance to Sustiva, I am currently on Sustiva and Trizavir, it also showed some resitance to AZT. I asked for another test from Virilogic so I could have both kinds of resitance tests done, as the first test was not the Pheno type. When this test came back they were unable to do a Pheno type because my viral load was undetectable, but it still had the other type of resitance test which was similar in results to the first test. My Dr wants me to stop Sustiva and go on to a PI called Atazanavir along with the Trizavir. I showed the same results to another HIV Dr. I have seen for a few yrs and he thinks I should continue on my current regiment since my resistance to Sustiva is just one log out not 10 logs ( not that I completly understand all this) I am sure all Dr's have different ways to look at the results from resistance testing and have different opions. Do you have any advice for a confused patient???

Response from Dr. Sherer

Even when we use special research techniques to sequence virus at extremely low levels, e.g. < 3 copies/ml, we find mutations. These are generally 'polymorphisms', which means single point mutations in small amounts that do not represent the genotype of the dominant species in a person...because the viral load is extremenly low, and that virus is not exhibiting the behavior of a virus that is resistant to the patient's regimen, i.e. it is still below detection.

So that may be similar to your situation. A key question in the situation above is, if one of the mutations is known to be associated with resistance to one drug in the regimen, e.g. the K103N for Sustiva, does that mean that that patient will eventually fail that regimen? Not necessarily, for reasons we don't understand. The fact that both mutations were seen on both genotypes increases the chance that they will persist.

The difference in the two opinions you received is understandable. The first doctor is aggressive and wants to avoid any chance of a virologic failure after seeing these mutations, so he/she is advocating an immediate change. The second doctor is conservative and wants to avoid changing to a different medication, and the possiblity of new side effects, since your current regimen is still suppressing your virus to below detection, and may continue to, in spite of the result from this genotype.

And finally, not all mutations are equal with Sustiva. Some cause partial resistance and some cause total resistance; you likely have the former.

My own opinion is consistent with the second doctor; I would continue the current regimen and monitor closely. There are a number of treatment alternatives that can be considered in future.


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