[Methods for determining viral tropism: genotype and phenotype tests].

The recent approval of the first CCR5 antagonist, Maraviroc (MVC, Celsentri), with specific antiviral activity against R5-tropic virus variants has generated the need for studies to determine the viral tropism in all those patient candidates for starting treatment with this new drug. Although genotyping methods appear to be the most useful tool due to its speed and simplicity, in the case of viral tropism, phenotyping techniques are currently considered the most reliable. In the last few years, different phenotyping assays have been developed to determine the use of the co-receptor. However, the Trofile phenotype assay is currently the one most used for the determination of tropism, since it is the only one that has been clinically validated. Given that the presence of X4- tropic variants in the viral population has been associated with virological failure to MVC, the main challenge of both genotyping and phenotyping tools is to optimise their sensitivity for detecting X4-tropic variants present in a minority of the viral population. At the same time, the correlation between genotyping/phenotyping methods is being evaluated to determine whether genotyping tools can be useful to make therapeutic decisions.