LONDON (AP) - New research indicates that performing a blood test after six days of new medication, instead of the typical four weeks, could get HIV patients onto the best drug cocktail more quickly, sparing them unnecessary side effects and reducing the virus' ability to become resistant to the pills.

The approach, described this week in The Lancet medical journal, accurately predicted 99 percent of the time when specific medications would not work for a particular patient in the long term.

Experts said that while the one-week test may turn out to be useful in three or four years' time, it is too soon to tell whether it could work for everybody.

The researchers, from the U.S. National Institute of Allergy and Infectious Diseases and the National Cancer Institute, analyzed medical information on 124 HIV patients that was collected during previous studies conducted between 1 97 and 2000.

The group included 90 children who were getting a single drug and 34 adults given a combination of four medications. The amount of HIV was measured in the blood of each patient daily for three months.

The findings indicated that the speed at which the virus disappeared from the blood during the first week of therapy predicted how effective a certain drug regimen would be for an individual.

The researchers found that when viral levels had dropped at day six to at least one-fiftieth of the original amount, the patients almost always responded well to the medication over three months.

However, if they had dropped only to one-fifth of the starting amount, or worse, the patients almost always did poorly on those drugs and the virus concentration rebounded three months later.

Those two thresholds together were accurate more than 95 percent of the time, the study said.

"This is preliminary," said Dr. Martin Hirsch, director of clinical AIDS research at Massachusetts General Hospital in Boston and a professor at the Harvard School of Medicine. "Their study suggests that you're not compromising anything by doing it earlier. However ... their srial was primarily in children and they also used regimens that aren't really conventionally used today."

"If you get a failure at one week, I would agree that something's going wrong. I would be tempted to switch," Hirsch said. "But I wouldn't quit at one week if there was some drop in the virus, however small."

There are about 17 different antiretroviral drugs to fight HIV. When someone starts taking drugs for HIV infection, it is important to know as soon as possible if the medicines are adequately suppressing the virus. If they are not, some patients can develop drug-resistant strains of HIV in a matter of weeks, reducing the chance that a new drug regimen will be successful.

Experts estimate that nearly 20 percent of new HIV infections today show some degree of resistance to current drugs.

"This will best be used in early, first-line therapy or second-line therapy. But in third-line therapy, which many patients are now coming to because they've been on these drugs for a long time, there will always be some compromise in how well the drugs work," predicted Dr. Deenan Pillay, director of Britain's Public Health Laboratory Service Antiviral Susceptibility Reference Unit at Birmingham University.

"In that situation, it's more difficult to imagine how you would use that six-day test because you don't know what to expect in the virus decline when you've already got some baseline resistance," he said.

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