WASHINGTON  A new analysis of the best time to begin HIV treatment found that starting early sharply improves survival, doctors said Sunday.

Doctors say the new evidence is certain to prompt many doctors to change the way they treat patients, and to prompt health officials to begin examining the evidence underlying guidelines for treating the AIDS virus. The study of 8,374 patients in the USA and Canada showed that those treated later in the course of HIV infection are 70% more likely to die than patients treated sooner, says lead researcher Mari Kitahata of the University of Washington-Seattle.

What makes the finding so striking, Kitahata says, is the "magnitude" of the survival difference between the two study groups.

"Seventy percent is a significant and substantial increase in the risk of death," she says.

The finding surfaced from the biggest comparison of the two treatment strategies ever carried out. The data were drawn from 22 studies from 1996 to 2006 in an attempt to answer a decade-old question about when to begin HIV treatment.

"The guidelines committees are certainly going to look hard at these data next time they meet," says Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, which sponsored the research.

Current guidelines say that patients should begin treatment only when their levels of a type of white blood cell called CD4 T-cells fall below 350 per cubic millimeter. HIV targets these cells, which in healthy people throttle up the immune response. But the guidelines have never adequately been tested, Kitahata says.

The new research found that patients fare better when they begin treatment when their CD4 counts are much higher, between 350 and 500 cells per cubic millimeter.

"There's reason to believe you would have even better survival using drugs available now," she says.

Early treatment, however, depends on awareness. Studies show that fewer than four in 10 U.S. adults have been tested for HIV.

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