- genome sequencing at baseline revealed resistance in minority species (Antivir Ther 2003;8: S150).
- A viral load of 500 to 1,000 c/mL is usually required.
- Genotypic resistance testing, is preferred for baseline testing, for early virologic failures, and for patients who have discontinued therapy. Phenotype and virtual phenotype assays may have advantages in patients with more extensive resistance, especially for assessing PI susceptibility. The relative merits of the available assays are discussed below ("Test Methods").
- Expert interpretation improves results.
Indications for Resistance Testing
Top HIV Med 2008;16:62; www.iasusa.
INDICATIONS:
(DHHS Guidelines for the Use of Antiretroviral Agents in HIV-1 Infected Adults and Adolescents, January 29, 2008; http://www.aidsinfo.nih.gov)
Most guidelines recommend resistance testing at baseline and in chronically infected patients failing ART.
- Primary HIV infection
- Chronic HIV infection: At the time of diagnosis, regardless of whether ART will be initiated.
- Virologic failure
- Failure to decrease VL >0.5 to 0.7 log10 c/mL by 4 weeks.
- Failure to decrease VL >1 log10 c/mL by 8 weeks
- VL >1000 c/mL after 16 weeks to 24 weeks
- Evaluation at time of virologic failure: A meta-analysis of published reports and conference presentations through February 2001 compared virologic results at 3 to 6 months after using resistance testing vs clinician decision or standard of care (SOC) for salvage regimens (HIV Clin Trials 2002;3:1). There was a modest but statistically significant benefit with genotypic testing. Results were significantly better with expert interpretation of the resistance assay. There was no significant benefit with phenotypic resistance testing compared with SOC. However, these studies were performed before the availability of clinical cut-offs (fold-change cut-offs derived from clinical trials) for many agents. In addition, fewer agents were available for patients with significant drug resistance, making the options for salvage therapy more predictable and less effective, regardless of whether resistance test results were considered. Viral suppression at 6 months after regimen change based on aggregate data for genotypic analysis vs SOC was as follows: genotype 168/432 (39%) vs SOC 115/400 (29%). Many studies showed only short-term benefit (AIDS
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