HIV drug resistance testing can perform well, even when viral load is as low as 250 copies/ml, results of two studies published in the online edition of Clinical Infectious Diseases show. In one study, genotyping was successfully performed on over 90% of samples when viral load was between 250 and 999 copies/ml.
Moreover, resistance testing at low-level viraemia was shown to be of clinical benefit, with results accurately predicting the risk of an increase in viral load to above 1000 copies/ml and the virological failure of antiretroviral therapy.
“Each of these studies provides a robust practical experience with data from a large number of subjects and plasma samples,” writes Dr Douglas D Richman of the University of San Diego in an editorial. “They provide evidence for beneficial clinical outcomes as a result of…drug resistance testing at low levels of plasma HIV RNA.”
To help guide the choice of antiretroviral drugs, it is recommended that people with HIV should have a resistance test before starting or changing treatment. However, approved genotypic resistance tests require a viral load of at least 1000 copies/ml. Their performance at lower viral loads is uncertain.
Investigators in Canada and Italy designed separate studies examining the reliability and clinical utility of standard genotypic resistance tests in cases of low-level viraemia – a viral load between 50 and 999 copies/ml.
The investigators in Canada retrospectively studied 4915 samples collected from 2492 people who received care in British Columbia between 1996 and 2012.
Overall, 88% of resistance tests conducted when viral load was below 1000 copies/ml produced usable results. Successful results were obtained from three-quarters of samples with viral loads below 250 copies/ml and from 90% of samples where viral load was above 250 copies/ml.