The battle against Aids could be boosted by a test developed by British scientists.

They believe they have cracked the problem of the killer virus "hiding" itself in the body, which can prevent effective treatment.

Doctors at Hammersmith Hospital, in London, say the new "circle test" could lead to drugs which would control HIV and prevent the onset of Aids.

The ability of HIV to disguise itself and go undetected in organs such as the brain, eyes and testicles, has proved an obstacle to effective treatment.

But the Hammersmith team, led by Dr Sunil Shaunak, has discovered an HIV "calling card", which tells them when the virus has gone into hiding.

It consists of small loops or circles of viral DNA, the waste products of replication, which can be found in white blood cells.

Treatment tailored

By detecting and tracking the circles it is possible to show the virus is alive and replicating in a patient taking anti-retroviral therapy. Treatment can then be tailored to the patient to prevent further replication.

Dr Shaunak, who heads a team from the Imperial College School of Medicine, said: "The 'circle test' will undoubtedly mean better treatment for patients with HIV and should help us to identify new therapies which will lead to total control of this virus within the body.

"The complex and toxic medications that HIV patients receive at present can now be monitored more effectively and should lead to more informed decisions about the best therapy for each patient."

Combination anti-retroviral therapy has successfully controlled HIV in some patients, but a cure for Aids has not been found.

'Crucial finding'

Dr Shaunak's team took blood samples from 63 patients in the UK and the US who had been taking anti-retroviral drugs for over a year and who appeared to be clear of HIV, reported the journal Nature Medicine.

Using the new test, 75% of the patients were found to have viral circles, and Dr Shaunak's team showed they still had the infectious virus in their bodies.

Lord Winston, director of research and development at Hammersmith Hospital, said: "Such a crucial finding is destined to have international repercussions and benefit the millions of people living with HIV around the world."

Gavin Hart, spokesman for the National Aids Trust, said: "It is an important step forward which should enable doctors to develop better and better treatments to fight the virus where it hides in the body."

But he warned the test was unlikely to be available in the developing world where most cases of HIV are found.