Info

Relative Advantages

Relative Limitations

Genotypic

Ease of availability Shorter time to results (days) Less technically demanding Mutations will likely precede phenotypic resistance Less costly when compared to phenotyping

Indirect measure of susceptibility May not correlate directly with phenotype Expert interpretation required Insensitive for detecting minor species Reliance upon known mutations in mapped areas of the HIV genome Lack of laboratory standardization

Phenotypic

Direct measure of susceptibility More familiar reporting results (IC50 or IC90)

Limited availability

Longer time to results (weeks)

Technically demanding

Insensitive for detecting minor species

Clinically significant breakpoints undefined Lack of laboratory standardization Costly

Genotypic or phenotypic testing for drug resistance before the initiation of antiretroviral therapy in treatment naïve patients cannot be routinely recommended at this time because of testing limitations and cost (52). However, the growing recognition of primary acquisition of resistant viruses, especially in certain parts of the United States, has led some experts to consider resistance testing prior to starting therapy in treatment-naive patients. Decisions regarding the initiation of therapy should always be made based upon patient-specific factors such as viral load, CD4+ cell count, and clinical status. Resistance testing should never delay the initiation of post-exposure prophylaxis in cases of occupational or nonoccupational HIV exposures.

Viral resistance has been linked to treatment failures, but it would be premature to advocate the routine use of resistance testing as a parameter for recommending alterations in existing antiretroviral therapy. Antiretroviral therapy should be changed when a previously undetectable viral load value has increased, and with a concurrent assessment of other factors such as vaccinations, illnesses, and adherence. Selection of new antiretroviral agents should be based on cross-resistance data available in the literature so that therapy is maximized. Until controlled trials provide more insights into use of testing and a consensus is reached on interpretation, clinicians should continue to rely on viral load and CD4+ cell counts to guide most therapy decisions.

The initial excitement that heralded the advent of antiretrovirals has been tempered by the emergence of resistance. HIV has demonstrated a tremendous capacity to mutate and evolve, making it a constantly moving target. Resistance continues to be the greatest impediment to the efficacy of the antiretrovirals. Trials are being conducted to discern the clinical implications of specific resistance patterns. Until a vaccine is developed, studies must continue to explore the clinical utility of genotypic and phenotypic testing. In the meantime, clinicians should rely on validated assays, ensure vigilant adherence, and use available resistance data to optimize antiretroviral drug selection.

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