Health officials in North Carolina have found a cost-effective way to identify people whose infection with the AIDS virus is so recent that the standard screening test would not normally work.

A new system pinpoints carriers earlier in the course of the disease, when they are at least 10 times more infectious than those detected by the standard test, researchers said in a study to be published on Thursday in the New England Journal of Medicine.

“This form of testing should be a standard tool for the prevention and surveillance of HIV infection and for the care of infected persons,” the researchers reported.

The system involves combining blood samples from 90 patients so that a highly sensitive - but expensive - test known as nucleic acid amplification can look for traces of HIV, the virus that causes AIDS.

If HIV is found in a batch, smaller batches are tested until technicians pinpoint which person is the carrier.

Over a one-year period, the system allowed researchers to find 23 additional HIV cases out of 109,250 people who were at risk for AIDS, an increase of 4 percent. There were two false positives.

All 23 people appeared to be healthy. One was pregnant, and because health officials got treatment to her early, her baby was not affected.

“If you’re looking to understand why the health department in North Carolina has hit a home run, that’s it,” chief author Christopher Pilcher of the University of North Carolina told Reuters.

Pilcher said thanks to the new system, the state is able to dispatch health workers sooner and prevent people from getting the virus who are exposed to people who are more contagious and at a stage in their life when they are engaging in high-risk behavior.

“You save hundreds of thousands of dollars when you prevent a transmission, and that’s just for one infection,” he said.

Nucleic acid amplification is routinely used by doctors to track the infection in people who have already contracted the AIDS virus. But when it comes to routine screening, the cost of that test adds up.

Health officials usually rely on the standard HIV screening test, which looks for antibodies made in response to the infection. It may miss 4 percent of the HIV cases, but it costs less than $10.

By contrast, nucleic acid amplification is far more sensitive because it directly measures HIV virus, but it costs at least $60 per test.

The pooling system would add $3.63 to the price of screening each person, the researchers calculated.

Other states may be slow to adopt the North Carolina testing strategy, Pilcher said, because it requires a new mind-set for laboratories, which normally focus on confirming positive results, not screening negative blood samples to look for newly infected people.

“There are two decades of tradition on how HIV testing should be performed. What we’re really recommending is turning the paradigm around, where the role of the laboratory is not to do antibody screening and confirm the positive, but do antibody screen and then confirm the negative,” Pilcher said.