Pat Bucy reports here that despite having undetectable viral load in blood (plasma)

25/25 subjects were found to have <10 to 4000 copies/ml HIV-RNA (mean 217 copies/ml) in their blood lymphocytes (PBMCs). They used a sensitive PCR test. The number of infected cells was low and the size of this pool of virus does not appear to change.

PBMC from a cohort of chronically HIV-1 infected patients on HAART for at least six months were examined for the presence of unspliced HIV RNA using a sensitive QC-RT-PCR assay. With a single exception, all 25 subjects examined had detectable vRNA in the blood lymphocytes concurrent with undetectable plasma vRNA. The geometric mean of vRNA across all subjects was 475 copies/10^{6} PBMC with a median value of 217. The range of values in individual subjects, however, was quite large (<10 to 4000 copies/10^{6} PBMC). Ongoing longitudinal analysis of these subjects indicates significantly less variation over time for each individual relative to the substantial cross-sectional variation for the overall cohort. A limiting dilution analysis (LDA) of these PBMC demonstrates a very low frequency of individual cells, each with approximately 1000 copies of vRNA per cell. The amount of vRNA in the bulk specimen of PBMC is in agreement with the amount predicted from the LDA determination of vRNA+ cell frequency and copies of vRNA in each cell. The frequency of cells detected by LDA/QC-RT-PCR analysis also correlates well with the frequency of vRNA+ cells detected by in situ hybridization of PBMC. These data indicate the persistence of a low frequency of transcriptionally active cells in the circulation, rather than low levels of cell-adherent virions, in most subjects with undetectable plasma virus on HAART.