What is pheno testing and what is it use for?

A phenotype is a drug resistance mutation test that measures various concentrations of each available ART drug against your virus in the laboratory. It is reported as the degree of susceptibility of your virus to each drug, compared to a fully susceptible or 'wild type' virus.

So a normal phenotyp in response to abacavir would be 1.0 fold, i.e. the same susceptibility as wild type virus (which is known to be free of ABC mutations), and a resistant virus would have, for example, a 6 fold reduction in susceptibility compared to wild type.

A genotype resistance test literally provides the amino acid sequence of the virus, and compares that sequence in wild type to the sequence in your virus. A fully susceptible virus is reported as 'no resistance mutations found' compared to wild type. An abacavir-resistant virus, for example, would be reported as (again as an example) the finding of these NRTI mutations: M184V, K65r, L74V, Y115F, and other thymidine associated mutations.

Both the genotype and the phenotype tests are used to assess the susceptibility of a patient's virus to their current treatment regimen, or to determine whether an acquired virus contains treatment mutations. The genotype is usually the first test ordered, and is sufficient for early failures and for tests of acquired drug resistance. With more complex treatment histories, the phenotype can add useful information and is often obtained with a genotype.