How to interpret resistance tests

Resistance test results are complex. To make this easier a summary report should also be included. This will say whether each drug is sensitive, intermediate or resistant.

Genotype results: 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:number: letter. For example. K103N. If you have this mutation, efavirenz is resistant and will not work.

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. This is like junction numbers on a motorway. In this example 103 refers to the 103rd amino acid in the RT section of HIV.

The final letter stands for the new amino acid. The N stands for asparagine.

Some mutations like K103N are easy to interpret. Others are harder, especially if there are many other mutations at the same time. Some mutations only have a small effect. Some are more rare. Some only occur in combination with others.

The Stanford Resistance Database includes charts for every mutation.

Phenotype results: given as a fold-change in sensitivity

Phenotype tests are now rarely used. The results have different cut-off values for each drug. They are also different for each make of test.

A 4-fold change in sensitivity is also called 4-fold resistance. This 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 August 2021.