CHICAGO (Reuters) - A simple device for detecting carbon monoxide in the blood may help doctors get an honest answer out of patients who smoke, U.S. researchers said on Monday.

The device, called a pulse co-oximeter, is typically used to test for carbon monoxide levels in firefighters, but it can also detect carbon monoxide levels in people who smoke, offering a powerful tool for educating patients about the effects of smoking.

"We were trying to just solve a little problem," said Dr. Sridhar Reddy, a lung specialist in St. Clair, Michigan, who presented the study at a scientific meeting of the American College of Chest Physicians in Chicago, along with his 16-year-old son Ashray.

"There is no good way to screen people for smoking," said Reddy, who encouraged Ashray to take on the study as a school science project.

"You can ask them directly, do you smoke. But once they say they don't smoke and they lie about it, they will never volunteer that information," Reddy said in an interview.

Reddy was looking for a quick, convenient method to detect whether a person smokes. Current tests involve breath, blood or saliva samples, but the pulse co-oximeter simply involves placing a clip-like device on a finger tip.

The pulse co-oximeter reads percentages of poisoned blood through a light that is shined through the finger nail.

Reddy's son Ashray wanted to find out how much carboxyhemoglobin -- blood poisoned by carbon monoxide -- would indicate whether a person is a smoker.

Working with his dad, he devised a questionnaire to determine patients' smoking habits. Meanwhile, Reddy recruited 476 patients in his office to take the test.

Together, they determined that patients with blood carbon monoxide levels of more than 6 percent were smokers, a finding Ashray confirmed through his patient surveys.

Ashray and his dad think the device might be a cheap, easy way to help doctors talk to their patients about smoking.

Reddy said most patients know what carbon monoxide is, and they respond strongly when they find it is circulating in their blood.

Reddy now routinely uses the test as part of a patient work-up. And instead of asking whether a patient is a smoker, he presents the test results and asks whether the finding could be related to smoking. His hope is this can become part of routine screening.

As for Ashray, the study earned him a medal at the local science fair.