British scientists report that dogs can be trained to detect bladder cancer in people by smelling their urine. The animals' exceptional sense of smell can distinguish the odor of compounds that tumors are thought to produce.

Man's best friend, the dog, has long been employed for purposes such as tracking the scent of missing humans or detecting the presence of illegal narcotics passing through airports. Now a new study by British scientists shows that dogs can be used to determine if humans have bladder cancer by sniffing their urine.

The research, led by Carolyn Willis at Amersham College in Buckinghamshire, is based on the theory that cancer produces chemicals with distinctive odors.

"It's probably a mixture of volatile organic chemicals, and probably not just one or two, but a combination of these volatiles that the dogs have learned to key into," she explains.

Current bladder cancer diagnosis requires a tissue sample to be removed, an invasive procedure that could be avoided with the use of dogs.

The researchers trained six dogs of various breeds and ages to discriminate between urine from patients with bladder cancer and from patients with other diseases. In the final tests, each dog smelled seven urine samples, one from a bladder cancer patient and six others from non-cancerous patients of the same sex.

As a group, the dogs correctly selected bladder cancer urine on 22 out of 54 occasions, an average success rate of 41 percent. This is three times better than untrained dogs would have performed by chance alone. The two best dogs, cocker spaniels named Tangle and Biddy, were right 56 percent of the time. Ms. Willis says this proves the principal that dogs can detect bladder cancer in urine, but she admits that the percentages are not yet high enough for diagnostic purposes.

"We showed that even dogs that aren't regarded as particularly brilliant scenting dogs can be trained as well to do this reasonably well," she adds. "We are going to be looking at ways to increase the accuracy rate by optimizing the training regime and the type of dogs that go into the training program."

The scientists undertook the study after two anecdotal stories published in a medical journal told of dogs that showed persistent interest in moles on the legs of their owners. The moles turned out to be cancerous.

An Imperial College of London professor of medical statistics named Tim Cole says he was delighted when he first learned of the latest research on bladder cancer, because it seems a logical extension of dogs' keen sense of smell. But he wonders if ultimately machines might be more reliable.

"It did actually flag up [signal] the fact there is something in the urine that can be detected," says Mr. Cole. "So if it can be detected by a dog, it could in theory be detected by the relevant machines that do these things, mass spectrometers. But it might be, spurred on by these results, that people would look for the active ingredients in urine, and once they had found them, I think that would make the whole thing much more straightforward in terms of routine detection."

Carolyn Willis says the ultimate goal is to develop instruments that can detect bladder cancer from urine samples. She notes that dogs are an interim step in the process of determining what chemicals bladder tumors give off into the urine. But she points to some instances where the animals could continue to be useful in this regard.

"There may be some uses for dogs, perhaps, in the Third World where these instruments are not available," says Ms. Willis. "Dogs in that situation might have a direct diagnostic use, but it is not ultimately the way that we would see things going in Western medicine."

The research by Ms. Willis and her colleagues appears in the British Medical Journal.