Genetic mutations in the HIV virus can confer drug resistance

It is more sensitive than existing methods, meaning resistant viruses can be detected at very low levels in patients' blood.

This could allow doctors to give patients the most appropriate treatment earlier than is currently possible.

The test, developed by Duke University Medical Centre, North Carolina, is reported in the journal Nature Methods.

A simpler and more easily used resistance test will improve treatment outcomes for people living with HIV Roger Pebody

The HIV genes mutate very quickly so that most people infected with HIV have many different forms of the virus in their bodies.

In some cases the mutated strains can become resistant to HIV drugs, making treatment ineffective.

Professor Feng Gao, who helped develop the new test, or assay, said: "Which resistant viruses are at hand can have important implications for the successful treatment of that patient."

He said there are already tests available to test for drug-resistant strains in patients, but these are time consuming and can only detect resistant strains when they are present at high levels in the patient's bloodstream.

HOW THE TEST WORKS HIV genes with mutations known to be linked to drug resistance were identified Fluorescent tags were added to genetic material from patient blood samples Tags designed to stick to the mutated genes were green Tags designed to stick to the same places where the genes were nonmutated were red A computer programme counted how many molecules had each colour tag attached The test could identify single mutated viruses in the sample The test also detected viruses with more than one mutation

If the new test is developed for clinical use as Professor Gao hopes, it would enable such strains, even if present at only low levels, to be identified quickly in patients.

They could then be treated with the most appropriate drugs from a very early stage to prevent drug resistant virus particles building up.

Roger Pebody, treatment specialist for the Terrence Higgins Trust, said: "Current tests for resistance to HIV drugs are expensive and hard to interpret.

"This means that only one in three people are offered them, despite guidelines encouraging their use.

"If this research leads to a simpler and more easily used resistance test, it will improve treatment outcomes for people living with HIV."

The test can identify individual HIV particles with mutations

Professor Gao said the test may also be useful in helping researchers to understand the process of resistance development in patients, as it is not yet clear which combinations of virus strains patients need to carry to develop significant drug resistance.

He said: "Our assay can detect all types of drug resistant strains, giving a more complete picture than other individual tests, from one blood sample."

"We can monitor drug resistance over time, for example which resistant strains appear first, second, third etc. [This] gives a lot of information on the dynamics of how resistance actually works."