Clinical Manual for Management of the HIV-Infected ... - myCME.com

Resistance Testing
Background
As of mid 2006, 2 major types of resistance testing are
available: genotype tests and phenotype tests.
Genotype Tests
Genotypic testing works by amplifying and sequencing
HIV taken from a patient to look for mutations in the
HIV reverse transcriptase and HIV protease genes,
which are known to correlate with clinical resistance
to antiretroviral drugs. This test generally can detect
mutations in plasma samples with HIV RNA levels
>1,000 copies/mL. Species representing 20% or more
of the amplified product usually can be detected by
current techniques. Minor species may not be detected.
Resistance mutations acquired in the past, under the
selective pressure of a previous drug, may be archived in
minor species and remain invisible to genotypic testing.
These resistance mutations may reemerge and cause
drug failure, however, if the previous drug is used again.
A genotype test takes 1-2 weeks to complete. The results
are reported as a list of the mutations detected; most
reports also include an interpretation that indicates the
drug resistance likely to be conferred by those mutations
(see “Modifying Factors” below, for a discussion of the
limitations of resistance testing).
Genotype results can be difficult to interpret. A
thorough antiretroviral history and expert clinical
review, therefore, are necessary to put the results of
a genotype test in proper perspective and to identify
options for further treatment. A compilation of the
most common HIV mutations selected by the 3 classes
of antiretroviral agents is available at: http://hiv-web.
lanl.gov.
A “virtual phenotype” is a genotype that is compared
with a databank of patients’ samples that have been
analyzed by paired genotype and phenotype testing. The
patient’s genotype is matched to a banked genotype, and
the patient’s phenotype is then predicted based on the
phenotypes paired to the banked genotype. A virtual
phenotype can be completed in the same amount of
time as a genotype. Results are reported as a genotype
(listing the mutations detected) as well as a predicted
fold change in the 50% inhibitory concentration (IC50)
of each drug to the patient’s virus (see “Phenotype
Section 3—Antiretroviral Therapy | 3–17
Tests” below). The predicted susceptibility of the
patient’s virus to each drug is then reported, based on
biologic and clinical cutoffs.
Phenotype Tests
Phenotypic testing works by splicing the HIV reverse
transcriptase and HIV protease genes from a patient’s
virus into a standardized laboratory strain, which is
then grown in the presence of escalating concentrations
of antiretroviral drugs. The test measures the IC50 of
each drug against the virus in vitro. Results are reported
as fold-change in IC50, as compared with a drugsusceptible
control strain or with a previous test of the
same patient’s blood. The predicted susceptibility of
the patient’s virus to each drug is then reported, based
on what is known about the correlation between foldchange
in IC50 of that drug and clinical resistance. As
with genotypic testing, the phenotype may not be able to
detect resistance if the HIV RNA is low (