The sensor, slightly bigger than a coin, is inexpensive

The hues of a series of 36 dots detect lung cancer accurately in just under three out of four people with the disease, the researchers told Thorax.

The concept of a "gas fingerprint" for lung cancer is not new, but the kit is.

The sensor, which is slightly bigger than a quarter dollar or a two pound coin, is inexpensive and easy to use.

It could revolutionise the way cancer is detected and potentially save lives, say the Cleveland Clinic doctors.

Experts have known for many years that the chemical composition of a person's breath changes when they develop lung cancer.

Dogs - animals with a very keen sense of smell - are able to distinguish the breath of patients with lung cancer from that of healthy people, for example.

Volatile organic compounds

This is because lung cancer cells give off chemicals, called volatile organic compounds or VOCs, which are then breathed out.

In the past, scientists have used highly sensitive machines such as gas chromatography and mass spectroscopy to "read" these VOCs with extreme accuracy.

But the machines are expensive to use and require specially trained experts to interpret the results.

In comparison, the colour sensor is cheap and easy to read, say the researchers.

The spots on the sensor change colour according to the chemicals with which they come into contact.

The researchers used the colour sensor to test the breath of 122 people with different types of lung disease, including 49 with cancer, and 21 healthy people.

It was able to accurately predict the presence of cancer in just under three out of four of those with lung cancer, including very early tumours.

This is crucial because lung cancer is often silent in its early stages, making it difficult to pick up at a stage when it could be treated effectively, explained lead researcher Dr Peter Mazzone and his team.

"Ultimately, this line of investigation could lead to an inexpensive, non-invasive screening or diagnostic test for lung cancer," they explained.

Dr Jesme Fox, medical oncologist and medical director at the Roy Castle Lung Cancer Foundation, said: "There is a desperate need to get people diagnosed earlier.

"At the moment we rely on people coming forward with symptoms, or a suspect chest x-ray picked up purely by chance.

"In the UK our five-year survival for lung cancer is about seven out of 100.

"That's appalling. Within one year from diagnosis almost 80% are dead. That's because people are picked up when the disease is advanced.

"If you pick it up early these people have a good chance of survival.

"This breath test certainly looks promising, being easy to use and non-invasive."

She said the test would require more development before it could become available clinically.