Purpose of review: A variety of methods are available to determine HIV-1 co-receptor usage, commonly referred to as viral tropism. This article reviews recent data on phenotypic and genotypic assays of HIV-1 tropism.
Recent findings: Tropism assays are used to determine co-receptor usage of HIV-1 in patients who may be candidates for treatment with CCR5 antagonists. Phenotypic assays are used most often in the clinical trials of CCR5 antagonists, and are considered the 'gold standard' for comparison with other methods of tropism testing. Enhancements have allowed detection of a lower threshold of minor CXCR4-using species. When compared with phenotypic assays, genotypic methods have poor sensitivity but good specificity at detecting CXCR4-using HIV-1. Preliminary results from a recent comparative study suggest that some genotypic methods may perform as well as phenotypic tests in predicting virologic response to CCR5 antagonists. Several studies show that tropism testing may provide useful prognostic information regarding the risk of disease progression.
Summary: Understanding the characteristic of different tropism assays is important for their clinical use. Although phenotypic testing currently is favored, genotypic assays may be a suitable alternative in appropriate settings.