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TABLE 2-7:

Results of Resistance Test Results in Patients with Acute or Recent HIV Infection
Source Year N Any NRTI NNRTI PI
CPCRA-US1 1999-01 491 11.6% 7.8% 3.0% 0.7%
Europe Commission2 2002-03 1083 9.1% 5.4% 2.6% 3.0%
Canada3 2000-01 494 6.1% 3.1% 1.2% 1.2%
10 N. Amer. cities4 1995-01 377 11.6% 7.8% 3.0% 0.7%
U.K.5 2002-03 171 19.2% 12.4% 8.1% 6.6%
CDC survey6 2003-06 3130 10.4% 3.6% 6.9% 2.4%

1. CID 2005;40:468
2. Wensing, A. XVI Int. AIDS Conference, Aug. 2006, Abstr. TUAB 0101
3. JAIDS 2006;42:86
4. NEJM 2002;347:385
5. BMJ 2005;331:1368
6. Topics HIV Med 2007;15:150

 

Test Methods: There are two types of tests, genotypic and phenotypic assays. These are compared in Table 2-8 (J Antimicrob Chemother 2004;53:555; Top HIV Med 2008;16:89).

 

TABLE 2-8:

Comparison of Genotypic and Phenotypic Assays
Genotypic Assays  
Advantages* Disadvantages
  • Less expensive ($300 to $480/test).
  • Short turn-around of 1 to 2 weeks.
  • Well standardized
  • Good reproducibility
  • Possibility of virtual phenotype*
  • More sensitive for detection of mixtures, which can occur with emerging or disappearing resistance
  • Favored in comparative studies with failure of first or second regimens.
  • Detect resistance only in dominant
    species (>20%).
  • Interpretation requires expertise or use
    of algorithms that vary in their ability to
    predict susceptibility.
  • Algorithms may be incomplete,
    especially for new drugs.
  • Require VL >500-1000 c/mL.
  • Limited data on non-clade B virus
Phenotypic Assays  
  • Interpretation more straightforward and
    familiar.
  • Assess total effect, including mutational
    interactions.
  • Do not require data on genotypic
    correlates of resistance (advantageous
    with newer agents).
  • Reproducibility is good.
  • Advantage over genotype when there
    are multiple mutations.
  • Provide quantitative assessment of
    susceptibility
  • More expensive (usually $800 to $1000).
    High cost may affect reimbursement.
  • Report takes longer than genotypic
    assay.
  • Clinically determined thresholds not
    available for all drugs.
  • Detect resistance only in dominant
    species (>20%).
  • Require VL >500-1000 c/mL.

*Virtual phenotype is rapid, easily done, and less expensive than phenotypic assays; disadvantages are inability to perform when database is inadequate.
2007 © John G. Bartlett, M.D. All rights reserved.33

Chapter 2: Laboratory Tests

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