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Are genotypic testing and resistance testing the same thing ?
Jul 11, 2006

HI Dr,Sherer, I have just been diagnosed with HIV and HCV. My HCV Dr. did a genotype test to find out what type of HCV that I had (1a). I asked my HIV Dr. about a genotype test and he said that that was not necessary. He did start me on meds because of my high viral load, but he said that the genotype test was not needed because it did not matter the type of HIV that I had, that they just treated it and if one drug did not work that they would move on to another. My questions are: Do genotypis HIV test only tell the type of HIV much like the genotype test for HCV or is it a drug resistance test all in one ?I have read ,on your web page, that Genotypic resistance testing is now recommended before any HIV drugs are initiated, should I be pushing my Dr. for this test and is it one test or two? I also have a question about Phenotype testing, what is that??? Thank you for any help in understanding all of this.

Response from Dr. Sherer

The purpose of the genotype test for hepatitis C (HCV) is to determine the type of HCV with which you are infected. Genotypes 1 and 4 are more difficult to treat.

HIV genotypes are now recommended in all patients in the US before they are treated. The reason is that 8-10% of patients who have never been treated have one or more drug resistance mutations. In one recent study in New York, the number was twice as high. The HIV genotype is thus recommended to prevent starting ART to which the virus you acquired is resistant.

If your virus load is still above 1,000, you and your doctor could still check the genotype, which is technically difficult to perform once the virus load is below 1,000 copies/ml. If not, then you will observe whether or not you acquired a mutation that has an effect on your treatment by your viral load and CD4 response.

A phenotype test is another type of HIV resistance test. In contrast to the genotype, which actually measures the amino acid sequence, and is reported as such ("M184V" is a substition of methionine for valine at the 184 locus, for example). A phenotype actually measures your virus against increasing drug concenctrations, much like an antibiotic sensitivity test, and is reported in comparison to the susecptibility of 'wild type' virus, ie. HIV with no mutations. Hence a person with the M184V that confers resistance to lamivudine would be said to have a "20-fold" decrease in susceptibility to lamivudine compared to wild type, which we know to be associated with resistance.

Most often a genotype test is performed on patients who have never been treated, and phenotype tests are reserved for more complex, multiple resistance patterns.

I urge you to talk to your doctor about these questions and these answers.


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