The company began offering the test in June of 2002, but only recently have data been published determining when and whether the results are at all meaningful.
Example of Replication Capacity Percentage on the PhenoSense GT Report
|Replication capacity (RC) indicates the ability of the virus to replicate in the absence of drug. Range represents 95% confidence interval around RC measurement. 100% = median RC of wild-type virus.|
In Italy, researchers studied a group of 18 people who were about to change anti-HIV therapy. Viral load, CD4+ count and HIV RC were tested at the time of the therapy change and one year later. People in the study fell into one of two groups; those who had a sustained increase in CD4+ counts to greater than 100 (immune responders) and those who did not (non-responders). Twelve of the 18 were immune responders and six were non-responders. Immune responders had a significantly greater reduction in HIV RC, compared to non-responders, regardless of their CD4+ cell count or viral load at study entry. Immune responders were also more likely than non-responders to have a specific mutation (M36I) in their virus.
A larger study of 189 people in San Francisco reported that reduced RC helps explain what is happening in people who have sustained increases in CD4+ cell counts even though HIV remains detectable. The study also showed that viral load was an important factor in predicting who might do well despite an incomplete virologic response to therapy. People who had viral loads below 10,000 were most likely to have continued good health.
A couple of very small studies also indicate that measuring HIV RC right after infection might be meaningful. The studies measured people's HIV RC right after infection. None of the people were on anti-HIV drugs yet. The studies found that people infected with a version of HIV with reduced RC had less disease progression than those whose HIV reproduced more rapidly. Further research is needed to confirm these results in greater numbers of people. But the data so far imply that RC testing may become another useful tool to help people decide when and whether to start anti-HIV therapy.
What remains less clear is the degree to which RC test results will be a useful measure of a person's risk for disease progression when they've never taken anti-HIV drugs at all. The decision about whether to start anti-HIV therapy for the first time can be a difficult one. While there's consensus that people whose CD4+ cell counts drop below 200 should definitely be taking anti-HIV drugs, there is much less agreement for people with CD4+ cell counts of 200-350. The U.S. Guidelines for the Use of Antiretroviral Drugs recommend that 350 should be the starting point for treatment, while others prefer to delay starting therapy until it falls below 200. Of course, there are other factors that must be considered, such as a person's general health, viral load and how rapidly CD4+ counts are falling. However, it could be tremendously helpful if an RC test could more clearly identify those people most at risk of disease progression.
Like any new test, including viral load and resistance tests, the more people who use them and the more they undergo the scrutiny of research, the more useful they become. Replication capacity has promise, and future studies will certainly improve the test and our ability to use it.
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