Tumors are thought to produce volatile organic compounds that emit distinctive odors that dogs can pick up, even when present in minute quantities.

The study involved urine samples from 36 patients with bladder cancer and 108 control samples from patients with non-cancerous diseases, as well as healthy individuals.

Six dogs of varying breeds and ages were trained over a seven-month period to discriminate between urine from patients with bladder cancer and that from control individuals.

For the final test, each dog was offered a set of seven urine samples, comprising one bladder cancer sample and six sex-matched controls. Some controls were also age-matched, and most had some form of non-malignant urological disorder.

The dogs identified their chosen sample by lying next to it. Each dog performed a total of 9 separate tests.

Taken together as a group, the dogs correctly selected bladder cancer urine on 22 out of 54 occasions, an average success rate of 41% compared to 14% expected by chance alone. The dogs' capacity to recognise a characteristic bladder cancer odor was independent of other chemical aspects of the urine, such as the presence of blood.

"Our study provides the first piece of experimental evidence to show that dogs can detect cancer by olfactory means more successfully than would be expected by chance alone," say the authors.

They expressed hope that their work would aid others engaged in similar studies by providing a benchmark against which future results can be compared.