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Milton Keynes, United Kingdom(CNN) Lucy, a cross between a Labrador retriever and an Irish water spaniel, failed miserably at guide dog school. As she was curious and easily excitable, random scents distracted Lucy from her master's path, and it wasn't long before she was unceremoniously kicked out.

But her owners knew their smart dog held promise. They decided, if her nose was getting her into trouble (she was after all, bred to be a hunting dog), why not train her to sniff out something useful?

For the next seven years, Lucy learned to sniff out bladder, kidney and prostate cancer, and was even used in a study. Over the years, she has been able to detect cancer correctly more than 95% of the time. That's better than some lab tests used to diagnose cancer.

Now, Lucy is part of one of the largest clinical trials of canine cancer detection. A British organization, Medical Detection Dogs, has eight dogs sniff out 3,000 urine samples from National Health Service patients to see whether they can discern who has cancer and who doesn't.

Claire Guest is the CEO of Medical Detection Dogs. Her fox red Labrador, Daisy, caught her breast cancer six years ago when she was 45. "She kept staring at me and lunging into my chest. It led me to find a lump," Guest remembers.

The tumor was deep in her breast. Her doctors said that by the time she would have felt it herself, the cancer would have been very advanced.

"Had it not been drawn to my attention by Daisy, I'm told my prognosis would have been very poor," she said.

Dogs have two 'noses'

Dogs' powerful noses have 300 million sensors, compared with a human's measly 5 million. In addition, dogs have a second smelling device in the backs of their noses that we don't have, called Jacobson's organ.

That double smelling system allows trained dogs to detect cancer's unique odors, called volatile organic compounds.

It took humans thousands of years to figure this out. In 1989, doctors at King's College Hospital in London wrote in The Lancet about a woman whose dog persisted in smelling a particular mole on her leg. That mole turned out to be early-stage malignant melanoma.

Over the next 26 years, studies from France to California to Italy have concluded that dogs really can detect the smell of cancer.

Researchers in the current British study have set a particularly high bar. They want to make sure dogs are actually smelling cancer and not something else, such as old age or a particular set of symptoms.

In the study, dogs will circle a carousel holding eight evenly spaced urine samples, one from a cancer patient and seven from patients who don't have cancer. At least one of those seven samples will be from someone about the same age as the cancer patient who had symptoms of cancer but didn't actually have the disease.

Guest, whose group is running the study, said that if studies like hers continue to show the power of dogs' noses, the animals might one day be used in conjunction with existing diagnostic tests, not instead of them. Scientists might also design a machine -- an "electronic nose" -- that mimics a dog's powerful smelling abilities. "It's very feasible," Guest said.

Meet the cancer-sniffing dogs Claire Guest is the CEO of Medical Detection Dogs, a company that trains dogs to sniff out cancer. Her fox red Labrador, Daisy, caught her breast cancer six years ago when she was 45. "She kept staring at me and lunging into my chest. It led me to find a lump," Guest remembers. Ulric, a cross between a Labrador and a golden retriever, is a bit of a water baby. He loves to get into troughs of water that are left out for cattle and will retrieve his ball from the bottom of a stream. Lucy, a cross between a Labrador retriever and an Irish water spaniel, failed miserably at guide dog school. But she has performed astonishingly well at sniffing out bladder, kidney and prostate cancer. She detects cancer correctly more than 95% of the time. That's better than some lab tests used to diagnose cancer. Karry, who is one-quarter curly-coated retriever and three-quarters Labrador, was too excitable to be a medical assistance dog for diabetics, so all that energy is now focused on problem solving. She's learning to sniff out prostate cancer. Kiwi, a yellow Labrador, is a former guide dog. She's a fantastic problem solver. Any dog has a powerful sense of smell, but hunting dogs like Kiwi are more easily trained. Fifteen-month-old Florin is young, but she's already showing promise detecting cancer. She's a fox red Labrador, just like her aunt, Daisy. She was bred with the hope that Daisy's cancer-sniffing DNA would run in the family. Behind her, Midas, a white-haired Hungarian vizsla, was initially trained to detect bed bugs. Now, she sniffs out cancer. Midas is a diva. She sits on Guest's chair when she isn't there, and bosses other dogs around.

Dampened enthusiasm

But more then 25 years later, the awesome smelling powers of dogs has not met with commercial success.

Dr. Sheryl Gabram thinks she knows why. The Emory University surgeon published a study four years ago showing that a machine, like a dog, could reliably detect the smell of breast cancer from patients' breath samples. Excited about their success, her team sought money for more studies. They failed.

"We submitted everywhere and could never get it funded to move the research any further," said Gabram, the surgeon-in-chief at Grady Memorial Hospital in Atlanta and director of the hospital's AVON Comprehensive Breast Cancer Center.

The problem, she thinks, is that while cancer-smelling dogs (or machines) make for great headlines -- one of her own patients received a great deal of publicity -- it's not always easy to envision how sniffing out cancer could be used commercially.

"It would need a lot of years of study and a lot of development," she said. "It's still far from that. People just thought it was too massive to embark on." It's too bad, too, she said, because "I think it's an area of research that's still promising."

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