HIV genotypic resistance testing is used to determine whether an individual is infected with drug-resistant HIV. Testing detects the presence of one or more mutations in the human immunodeficiency virus (HIV) that are associated with anti-retroviral (ARV) drug resistance. This is done in order to guide drug treatment.
Genotypic resistance testing is essentially used as an evaluation of the HIV strain infecting an individual.
Testing may also be used to help determine the cause of drug treatment failure. ARV and highly active retroviral therapy (HAART) are intended to suppress the person's viral load in order to slow the progression of the disease and to minimize viral replication and mutation. Treatment failure is identified when a person's viral load begins to increase despite treatment.
HIV genotypic resistance testing may be ordered when a person is first diagnosed with an HIV infection to determine whether the strain of HIV is known to be resistant to certain ARVs, regardless of whether the person is intending to begin treatment.
It may also be ordered immediately before the start of therapy when a drug-resistant virus is suspected, so that appropriate therapy is given. Testing may also be ordered when HIV viral load values rise steadily during therapy, indicating treatment failure and the possibility of resistance. Generally, a treatment is considered a failure if a viral load rises more than three-fold in consecutive tests. If drug resistance is found, a new treatment regimen may be chosen.
Genotypic resistance testing should be performed on all HIV-positive pregnant women prior to starting therapy and for those women who become pregnant while on ARV and who have detectable viral loads.
In general, the mutations present indicate the drugs to which the HIV strain is likely to be resistant. This information helps a doctor predict which treatment regimen is best for a specific person. However, not all mutations cause drug resistance and the results of genotypic resistance testing must be interpreted with caution. Often, a specialist is consulted when considering results of the test and selecting appropriate ARV.
The test results identify the viral mutations. These are described by a combination of letters and numbers (for example, K103N) where the letters refer to the amino acid associated with the gene and the number refers to the position of the mutation in the genome.
The International AIDS Society—USA maintains a list of primary and secondary mutations that are most commonly associated with resistance to various ARVs and ARV categories. The list is intended to help interpret the results of resistance testing and in selecting appropriate ARV therapy.
Genotypic resistance testing is not recommended if a person's viral load is less than 500 copies/mL. In this case, there is insufficient amount of HIV genetic material (RNA) for testing to be reliable. Testing works best if the viral load is greater than 1,000 copies/mL but is recommended even if the level is between 500 and 1,000 copies/mL.
HIV genotypic resistance is a qualitative test and detects mutations associated with drug resistance. It does not report the level of resistance associated with the drug. Phenotypic resistance testing (see Common Questions #2) is a better indicator of the level of HIV drug resistance.
HIV genotypic resistance testing cannot detect unknown resistance mutations that may be present in various strains of HIV. The test may not detect a drug-resistant strain of HIV if the resistant virus makes up less than 10-20% of total virus that is present circulating in the blood.
Testing is not recommended after discontinuing a drug. This is because the percentage of drug-resistant strain may decrease considerably in the absence of the drug. When "selective pressure" from a drug is absent, the drug-resistant strain may drop enough to the point where testing would not be able to detect it.
This article was last reviewed on November 30, 2012. | This article was last modified on November 30, 2012.
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