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New Resistance Test Combines Phenotype and Genotype

By John S. James

November 23, 2001

On November 15 ViroLogic, Inc. announced a new testing service that combines phenotypic and genotypic resistance testing on a single report. The new test, named PhenoSense GT, is expected to be used especially for patients whose treatment is complicated by difficult or complex HIV resistance. The company expects to get test results back to physicians within 14 days.

Phenotypic resistance testing uses part of the patient's virus to construct a new virus that is then grown in the lab and tested with varying concentrations of the approved anti-HIV drugs, using automated equipment (it would be difficult to grow the unchanged patient's virus in a laboratory). Genotypic testing looks for mutations known to be associated with resistance to the various drugs; it is usually less expensive than phenotypic testing, but more difficult to interpret. While the results are often similar, the two kinds of tests can give different information in some cases.

The PhenoSense GT assay includes resistance testing for the newly approved drug tenofovir.

The number to call for ordering this test is 1-800-777-0177. Unfortunately the price is $1,210.

New information on HIV resistance will be presented at the ICAAC conference in Chicago in mid-December.


We believe that offering the two tests on one integrated report, by a single company that applies compatible standards and quality assurance throughout, opens opportunities to provide better information to physicians. How well it is accepted in practice remains to be seen.

As with other resistance testing, a major challenge will be giving physicians the help that they need to get the most out of the test. HIV physicians must keep up with many things, and cannot all be expected to be drug-resistance experts. At this time we do not know what information the company plans to provide to help physicians interpret the report.

What we would like to see eventually is a semi-automated system that would compare patterns in the reported data with a database of past experience, and add to the report any one or more of perhaps thousands of pre-written notes, calling the physician's attention to significant information that might otherwise be overlooked. Ideally, artificial intelligence software would make the first selection of these interpretive notes. Then an HIV-resistance expert or panel of experts would look at every report that was not clearly routine, changing the selection or text of the notes when there is a good reason to do so (these experts could work online from anywhere). Any improvements by the human expert(s) would go to the physician, and also be used to improve the software for the future.

ISSN # 1052-4207

Copyright 2001 by John S. James. Permission granted for noncommercial reproduction, provided that our address and phone number are included if more than short quotations are used.

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