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Overview of HIV drug resistance testing assays

Michael J Kozal, MD
Section Editor
Martin S Hirsch, MD
Deputy Editor
Jennifer Mitty, MD, MPH


The use of drug resistance testing has become an integral part of HIV clinical care. The first clinical description of HIV resistance to antiretroviral drugs was published in 1989 about patients taking zidovudine monotherapy; accumulation of mutations within the reverse transcriptase gene resulted in a marked increase in drug resistance [1]. Subsequently, HIV variants resistant to every available antiretroviral agent have been identified in viral culture in the presence of drug and in treated HIV-infected patients. The evolution of drug resistance has significant clinical implications for choosing effective antiretroviral regimens [2].

Resistance assays can be categorized as either phenotypic or genotypic. These assays detect resistance in fundamentally different ways, although the results generally correlate with each other. There may be clinical settings in which one or the other assay could offer theoretical advantages or in which each assay would provide complementary information. However, the optimal specific use of these assays has not been definitively established.

The clinically available antiretroviral drug resistance assays will be discussed here, including the benefits and limitations of each type. Clinical trials evaluating these assays and the clinical use of these drug resistance assays in various patient populations are discussed elsewhere. (See "Drug resistance testing in the clinical management of HIV infection" and "Primer on interpretation of HIV drug resistance testing".)


Phenotypic resistance assays measure the extent to which an antiretroviral drug inhibits virus replication in vitro. Similar to bacteriologic methods, this is typically performed by demonstrating an increase in the inhibitory concentration (IC) that is required to inhibit in vitro growth by 50 percent (IC50) compared with virus replication in the absence of drug. Results are reported as a fold-change in drug susceptibility of the patient sample compared with a laboratory reference strain.

Recombinant virus assay — Historically, phenotypic resistance assays were performed using virus cultured from patient peripheral blood mononuclear cells [3]. These assays, which required culturing virus in primary human peripheral blood mononuclear cells, were plagued by significant variability and lengthy turnaround times. They required approximately two to four weeks for initial HIV isolation, one week to titer the virus inoculum, and an additional one to two weeks to assay virus replication in the presence of different drug concentrations.


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Literature review current through: Jul 2017. | This topic last updated: Oct 07, 2014.
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