How to interpret resistance tests

Resistance tests can be complex to interpret, but also include a summary report on whether each drug is sensitive, intermediate or resistant.

Genotype results are given as letters and numbers.

Results from genotypic resistance tests are given as a list of mutations. These mutations are changes in the structure of the virus, usually where one amino acid has changed to another. The order of amino acids determines how the gene is able to function.

These usually follow the format of a letter followed by a number followed by a letter – i.e. K103N which results in complete resistance to efavirenz and nevirapine.

The first letter stands for the amino acid that is normally at that junction in the virus. The K stands for lysine.

The number says where on the HIV DNA that the change has taken place – like junction numbers on a motorway. In this example 103 refers to the 103rd amino acid in the RT section of the HIV genome.

The final letter stands for the new chemical that the mutation makes. The N stands for asparagine.

Some mutations like K103N are easy to interpret but most others are more complicated. This is because they may only have a small effect, or because they are more rare, or because they commonly occur whether or not you are on treatment.

The Stanford Resistance Database includes charts for every mutation.

Phenotype results are reported as a number (a fold-change in sensitivity)

Phenotype tests are now rarely used. The results have different cut-off values for each drug and for each manufacturers test.

A 4-fold change in sensitivity (also called 4-fold resistance) could mean complete resistance for one drug and complete sensitivity for another.

Luckily, phenotypic tests also include an interpretation report. This report summarises which drugs are still sensitive, which are partly resistant and which are completely resistant.

Last updated: 1 January 2018.