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A Closer Look: Virtual Phenotypic Resistance Test
Part of A Guide to HIV Drug Resistance

December 2009

Mark Holodniy, M.D., F.A.C.P., C.I.C.
Mark Holodniy, M.D., F.A.C.P., C.I.C., Stanford University; Veteran's Affairs Medical Center, Palo Alto, Calif.
Patient: Kyle C.
Doctor: Mark Holodniy, M.D.
CD4 count at the time of the test: 287
Viral load at the time of the test: 34,500
Current CD4 count: 361
Current viral load: undetectable
Resistance mutations: K20T, L33F, M36I, I54L, Q58E, L63P, T74S, L90M, K103N, M184V, K219R, L228R, K238N

Kyle C. (not his real name) has been on HIV treatment for a long time. His first regimen consisted of only Epivir and Retrovir. After two years on that regimen, he switched to his first triple-drug regimen: Combivir (a combination of Retrovir and Epivir) and Sustiva. At the time this resistance test was taken, Kyle was taking Epivir, Videx, Norvir and Invirase.

Although Kyle's overall health was good, his CD4 count was 287 and his viral load was 34,500, which indicated that his HIV medications weren't suppressing HIV as well as they should. His doctor, Mark Holodniy, M.D., ordered a virtual phenotypic test.

This test found that Kyle's HIV was highly resistant to many HIV medications: Epivir and Emtriva (both NRTIs), every NNRTI available at that time and many protease inhibitors, especially if given unboosted. To some medications, such as Videx, Ziagen, unboosted Crixivan or Prezista, Kyle's virus had developed some, but not complete, resistance. Other drugs remain fully active, such as the NRTIs Retrovir, Zerit and Viread. The protease inhibitors Crixivan, Invirase, Kaletra and Aptivus retained full activity, but only when boosted with Norvir. Although Kyle was on a regimen containing boosted Invirase, predicted on this test to have Maximal Response, his medications did not fully suppress HIV. This could be for several reasons, including the possibility that Kyle was finding it difficult to take all his medications on time.

Based on the results of the test, Dr. Holodniy prescribed a new regimen: Epzicom, Viread and Kaletra. Six months later, Kyle was doing extremely well: His CD4 count was up from 287 to 361, and his viral load became undetectable.

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A Closer Look: Virtual Phenotypic Resistance Test
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