Abstract: An assay to detect or quantify HIV infectious virus from clinically relevant cellular compartments, or reservoirs, in anti-retrovirally treated patients whose viral levels are low to undetectable is described. The method detects infectious virus in patients whose plasma viral loads are considered to be below the limit of current PCR based detection methods and thereby is more relevant for guiding treatment. A further advantage is that the method allows viral tropism to be directly determined in the presence of specific inhibitors of CCR5 or CXCR4. Drug sensitivity can also be directly determined without the need to laboriously recover patient virus by culture for extended time periods, a method that allows for viral selection or evolution, which is not desirable. Patient cells, like the blood mononuclear cells, or monocytes, are isolated and cultured in the presence of cytokines like CSF-1/M-CSF or GM-CSF. to promote their differentiation.