Combined phenotypic/genotypic testing of little value in those failing ART

Last Updated: 2005-05-02 16:40:44 -0400 (Reuters Health)

NEW YORK (Reuters Health) - The results of a controlled trial provide no clear evidence that the addition of phenotypic resistance testing to genotypic testing adds value in HIV-infected patients who are failing antiretroviral therapy and have limited therapeutic options.

But investigators urge caution in generalizing their findings, pointing out in the April 15th issue of the Journal of Acquired Immune Deficiency Syndromes, that it is "inherently difficult to assess diagnostic tests in a rapidly moving field and the results of this study largely reflect the effect of tests that have been superseded."

Current guidelines hold that resistance testing should be used for all HIV-infected patients experiencing treatment failure in whom a change in treatment is being considered. However, "guidance on the choice between genotypic and phenotypic testing is less prescriptive, partly because of the limited randomized evidence on this issue," they note.

It's also been suggested that both assays might provide "critical and complementary information."

To investigate further, Dr. David Dunn from the Medical Research Council Clinical Trials Unit in London and colleagues randomized 311 appropriate patients to genotypic resistance testing alone or to genotypic plus phenotypic testing.

"Patients were eligible if a decision had been made to switch antiretroviral therapy, the most recent HIV-1 RNA plasma viral load exceeded 2000 copies/mL, and the clinician was unable to select a potent regimen of three or more drugs without access to a resistance test," they explain.

The results showed "no substantive differences between genotypic testing alone and the combined use of genotypic and phenotypic testing in virologic or immunologic response," according to the team.

The added information provided by phenotypic resistance tests had no appreciable effect on the drug regimens prescribed after randomization or on the change in plasma viral load from baseline to 12 months, the primary study endpoint.

Moreover, stratification by prior drug history "failed to identify any particular subgroup that gained benefit from the addition of phenotypic testing," the team also reports.

J Acquir Immune Defic Syndr 2005;38:553-559.