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mutations
Aug 28, 2006

does the pheno test tell how much of the viral load are weak mutations and could a drug still be working on the remainder of the virus eventhough the test showed complete resistance to it?

Response from Dr. Sherer

While a genotype shows all of the mutations, without information on its impact on your virus, the phenotype shows the degree of susceptibility of your virus to each member of the regimen. So yes, the phenotype does show which mutations are 'strong', i.e. lead to complete loss of susceptibility of your virus, and which are 'weaker', i.e. may only cause little or partial loss of susceptitibility. This is one reason that many clinicians prefer a phenotype test in a patient with heavy treatment experience and many different mutations that may interact in unexpected ways.

There are some circumstances where a phenotype test showing 'complete' resistance is misleading due to the presence of some continued activity of the drug. For example, the phenotype for patients on 3TC who have the M184V mutation will show complete resistance, but the drug will still offer a 0.5 log viral suppression, even in the presence of that mutation.

Loss of activity of members of the NNRTI class like EFV (Sustiva) and NVP (mevirapine) is considered to be complete in the presence of the signature NNRTI mutations, such as the K103N and the Y181C, and their continued use in the presence of such mutations is not recommended.

In the majority of cases, a report of complete loss of susceptibility to a drug does in fact mean that it should no longer be used. Its not possible to generalize further on this subject; any further information will require your own physician and a specific genotype, phenotype, drug history, treatment outcome history, and toxicity history to sort out whether an individual drug may still have use in a given patient.

I urge you to talk to your doctor about your question and this answer.


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