Some of the variation in genital fluid viral load may be due to

Some of the variation in genital fluid viral load may be due to differences in the degree to which different ART medications enter genital fluid. Recently developed research methods allow for measurement of drug concentrations in seminal and vaginal fluids, which can then be compared to drug levels measured in blood. This research has found that some ART medications achieve higher concentrations in genital fluids than others [24-27]. For instance, nucleoside/tide reverse transcriptase inhibitors (NRTIs) penetrate to a greater extent in male and female genital secretions than do protease inhibitors (PIs). Further work of this type may eventually aid in selection of antiretroviral medications in order to reduce sexual transmission. However, more data collected via these methods and better understanding of the degree to which this approach might be effective is needed before specific recommendations can be made.

In addition to differences in viral load between plasma and genital fluids, there may also be differences in the resistance characteristics of virus in these two locations. HIV may become resistant to ART medications through mutations that occur during replication and through exposure to insufficient or inconsistent levels of HIV medications. This may happen when ART medications are not taken according to the prescribed schedule or doses are skipped. In addition, drugs which do not enter the genital fluid as well may help promote the development of resistance in the genital fluid specifically. Some researchers have noted that within an individual, the resistance characteristics of virus isolated from genital fluid may differ from those of virus isolated from plasma [28-30].