What is phenotypic testing?
Unlike genotypic testing, which looks for particular genetic mutations that causes drug resistance, phenotypic testing directly measures the sensitivity — or phenotype — of a patient’s HIV in response to particular antiviral drugs. Phenotypic resistance tests measure the concentration of a drug required to inhibit viral replication in the test tube by a defined amount such as 50% or 95%. This is called IC50 or IC95. IC stands for inhibitory concentration. In other words, a laboratory conducting a phenotypic test is trying to determine the amount of drug needed to stop HIV from reproducing. If it only takes a standard amount of the drug — a concentration equal to that used by HIV-positive people — HIV is not resistant to the drug. If higher amounts of the drug are needed to stop HIV from reproducing, HIV is considered to be resistant to the drug being tested. The concentration of drug necessary to inhibit virus replication is expressed in units called nanomoles (nM).
With Phenotypic testing-you take virus from a person and put it in a test tube and add increasing amounts of a drug. You want to see how much the virus is able to reproduce itself as you add increasing amounts of drug. And how much drug is required to stop virus reproduction. If all you need to stop HIV from reproducing is an amount of drug equivalent to the amount a person would take that person does not have phenotypic resistance and that drug should work for that person. If you need 4 times that amount of drug to suppress HIV from reproducing then your virus is 4-fold resistant to that drug. If you need 10 times the amount of drug than your virus has 10 fold resistance to that drug. Exception ABT-378 because drug levels are very high. Phenotypic testing actually measures more directly if you are resistant to a drug.
Phenotypic testing directly exposes a person’s virus to each of the currently available antiviral drugs. The virus is exposed to a specific drug, and then evaluated to determine how much of the drug is required to block viral activity. An individual’s virus is considered to be less susceptible to a particular drug when more of the drug is required to block the virus.
