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Updated October 21, 2008

Case 2: Features of Genotypic and Phenotypic Resistance Assays

Authors: Christopher Behrens, MD David H. Spach, MD Lisa M. Frenkel, MD

Learning Objectives

A 38-year-old HIV-infected woman has extensive antiretroviral therapy experience with several previous virologic failures. She is now seen for evaluation of a possible salvage regimen. Her CD4 count is 128 cells/mm3 and HIV RNA 35,350 copies/ml. An HIV resistance test is ordered.

Which one of the following statements is TRUE regarding HIV resistance testing?

A Compared with the phenotypic assay, the genotypic assay is more expensive and has a longer turnaround time.
B The M184V mutation notation refers to the replacement of the amino acid methionine (M) by valine (V) at amino acid position 184.
C The genotypic assay detects resistance in greater than 98% of minority populations of HIV that exist within an individual.
D When a patient is undergoing a change in antiretroviral therapy because of virologic failure, genotypic resistance testing does not provide any benefit in improving virologic responses.