Drug Resistance and Other Laboratory Monitoring Assays in HIV infection

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Although CD4 cell counts and plasma viral load assays are the principal laboratory tests used to monitor the progress of HIV-1 infections, several other assays are assuming increasing importance to adequately assess the benefits of antiretroviral therapy (ART) in infected individuals. The accessibility of such assays will vary greatly, depending on the resources available to treat HIV infections.

Where testing capability exists, HIV drug resistance testing is useful when patients enter care prior to initiating therapy and again when considering change of regimens during virologic failure. Genotypic assays are generally preferred because of cost and rapidity, except in situations where multiple ART regimens have been used, when phenotypic assays may be of value.

When abacavir is being considered as part of an ART regimen, genetic screening for HLAB* 5701 is helpful to reduce the risk of severe abacavir hypersensitivity reactions. These reactions, reported in 5-8% of white and 2-3% of black patients occur primarily in individuals with the MHC class I allele HLA-B*5701. Individuals who screen positive for HLA-B*5701 should not receive abacavir.

When a CCR5 antagonist (e.g., maraviroc) is being considered as part of an ART regimen, a coreceptor tropism assay is useful, since an agent of this class will only suppress viruses that utilize this receptor (R5 viruses). CCR5 antagonists should not be used in individuals who carry primarily X4 or dual/mixed tropic viruses. Currently, the principal assay available to measure HIV-1 tropism is phenotypic, though genotypic tests are under study.

Although therapeutic drug monitoring is recommended by some, its use remains controversial, and no clear-cut recommendations can be made regarding its utility.