WASHINGTON -- New tests that promise to speed up diagnosis of food poisoning pose an unexpected problem: They could make it more difficult to identify dangerous outbreaks like the one that sickened people who ate a variety of Trader Joe's peanut butter this fall.

The new tests could reach medical laboratories early next year, an exciting development for patients. They could shave a few days off the time needed to tell whether E. coli, salmonella or other foodborne bacteria caused a patient's illness, allowing faster treatment of sometimes deadly diseases.

The problem: The new tests can't detect crucial differences between different subtypes of bacteria, as today's tests can. That fingerprint is what states and the federal government use to match illness to a contaminated food.

"It's like a forensics lab. If somebody says a shot was fired, without the bullet you don't know where it came from," said Dr. Phillip Tarr of Washington University School of Medicine in St. Louis.

The federal Centers for Disease Control and Prevention expects private labs to adopt the next-generation tests rapidly, and warns that what is progress for individual patients could hamper national efforts to keep food safe.

Already, 1 in 6 Americans gets sick from foodborne illness each year, and 3,000 die.

So even before these tests hit the market, the agency is searching for solutions. The CDC's Dr. John Besser said the tests' unintended consequence could be that ultimately, more people become sick.

"In the past 20 years, there's been a fantastic ability to fingerprint bugs: Is this an organism that's causing multiple infections and we can interdict it? Or is this a once-only event?" said Tarr, an E. coli specialist. "Without that organism in hand, the . . . [governments] can't do it. You lose the ability to get the evidence."

It comes down to what's called a bacterial culture, whether labs grow a sample of a patient's bacteria in a petri dish, or skip that step, not required by the new tests.

Here's the way it works now: Someone with serious diarrhea visits the doctor, who sends a stool sample to a private testing laboratory. The lab cultures the sample, growing larger batches of any lurking bacteria to identify what's there. If disease-causing germs such as E. coli O157 or salmonella are found, they may be sent on to a public health laboratory for more sophisticated analysis to uncover their unique DNA patterns -- their fingerprints.

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Those fingerprints are posted to a national database, PulseNet, that the CDC and state health officials use to look for food poisoning trends. But culture-based testing takes time -- as long as two to four days after the sample reaches the lab, which makes for a long wait if you're a sick patient.

Just this fall, PulseNet matched 42 different salmonella illnesses in 20 different states that were eventually traced to a variety of Trader Joe's peanut butter.