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The parallel allele-specific sequencing (PASS) assay was found to be 1,000 times more sensitive than tests currently used to determine drug resistance in HIV-positive patients.

The assay identifies specific gene mutations in HIV molecules known to refer resistance to antiretroviral drugs.

For the latest study, the PASS assay was used to analyse plasma samples from six HIV-positive patients who were receiving treatment that was unsuccessful, three who had previously received antiretrovirals and four who had not.

HIV genes were isolated from the plasma samples and exposed to fluorescent tags. Tags were labelled to appear green if attached to a mutant version of a gene linked to drug resistance and red if attached to a normal version of the gene.

A microarray scanner was then used to view the tags and computer software counted the number of molecules with green or red tags in each sample.

For example, tags were used to identify HIV viruses with the M184V mutations, which are known to cause resistance to the antiretroviral drug lamivudine.

In patients whose treatment had failed, 99 per cent of HIV molecules had this mutation. In comparison, the M184V mutant comprised 1.5 per cent of HIV molecules in those who had previously received treatment and none were found in those who had received no antiretrovirals.

PASS demonstrated a sensitivity of 0.01 per cent, compared with 20 per cent typically observed with the genotypic and phenotypic tests for resistance currently in use.

Lead researcher Dr Feng Gao, from Duke University Medical Centre in North Carolina, said: ‘Our test may serve as a useful tool for doctors to make an informed selection of which anti-HIV drugs they should treat their patients with to prevent quick development of resistance.