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HIV Drug ResistanceHIV Drug Resistance
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Sep 14, 2006

Dear Dr. Can mutations that showed up 8 years ago that no longer show up on current genotype test have disappeared completely or are they still lurking somewhere? If they still lurk then how valuable are the genotype tests anyway? Thank you.

Response from Dr. Sherer

Yes, mutations that were present 8 years ago should be considered to be present, whether or not they show up on a genotyope test. This is called 'archived mutations'.

They may not show up on a genotype test now, 8 years later, if there is no selection pressure on a patient's current strain of virus (due to a different ART combination) causing it to occur in the dominant species. This is an important concept to understand: Though the mutation is still present in a MINORITY species, it may not show up on the genotype test because the test is unable to measure viral species that represent a minority present in less than 20% of the population.

This is not a flaw of the test itself, its just another example of our needing to understand exactly what a test can and cannot tell you. In order to understand a patient's current viral susceptibility, therefore, an HIV doctor has to know not only the result of the test, but also their past ART treatment history, the response to treatment, toxicities, and the results of any past resistance tests to fully understand and interpret a single resistance test, and to make useful recommendations for subsequent treatment.

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