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If you have an infection, the diagnosis starts with a guess from your doctor. But there's new technology that throws out the guesswork. It searches for DNA circulating in your body and tries to match it with DNA from a wide variety of viruses, bacteria, even parasites. NPR's Richard Harris reports it's already been tried out in dozens of patients.

RICHARD HARRIS, BYLINE: One of those patients is a young woman from San Francisco named, Andrea Struve, who came back from 40 days in the Australian outback with a nasty set of symptoms.

ANDREA STRUVE: I was in classes, sweating profusely with a fever, joint pain, and it just wasn't fun. So that's when I went to the doctor.

HARRIS: Her doctor made a bunch of educated guesses about the underlying cause, but all the tests came back negative. So Struve was enrolled in a study at UC San Francisco to try out a whole new approach to diagnosing infection.

CHARLES CHIU: As opposed to the way we normally diagnose infectious diseases. Meaning that we target a single infectious agent at a time, this test works by detecting all the DNA present in clinical samples.

HARRIS: Doctor Charles Chiu, at UC San Francisco is running that study. He extracted DNA from her blood and ran it through a superfast sequencing machine. He compared the DNA he found with a huge library of DNA sequences from all sorts of infectious agents. It turned out that she was infected with a virus that normally causes a roseola rash in young children.

STRUVE: They're not entirely sure why I got it, but that was the reason I was so sick, which is kind of cool.

CHIU: But it must've been reassuring to learn that you just had some sort of garden-variety disease and not some bizarre tropical disease?

STRUVE: Yes, definitely. I was really, really glad it was something that would be gone in a month rather than six months to a year.

HARRIS: She's fine now. Her case is documented in the journal Genome Research. Chiu and his colleagues have solved an even more dramatic case, which they've written up for the New England Journal of Medicine. It involves a 14-year-old boy in Wisconsin with brain inflammation. Again, doctors ruled out everything they could think of with common tests.

CHIU: He was rapidly becoming critically ill. Within a few days we actually got the sample and within 48 hours we were able to run this assay and were able to detect an organism that was likely the cause of his infection.

HARRIS: It was a bacterium that can be treated with high doses of penicillin. And sure enough, the boy recovered after that intensive treatment. So far, of the 30 plus cases, Chui says they've been able to identify an infectious culprit about one quarter of the time. Some of the remainder turned out not to be infectious at all. The test is still experimental. It's done by hand in a university setting and it cost $1,000 just in labor and materials, not counting the pricey machinery. But Chui hopes in time it will be the go to analysis when traditional blood tests don't provide the answer. Richard Harris, NPR News.

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