Welcome back to ALL THINGS CONSIDERED from NPR News. I'm Jacki Lyden.

Scientists announced today they found a urine test that could determine which smokers will develop lung cancer and which will not. But the researchers say a clean test won't be an excuse to smoke.

NPR's Joanne Silberner has more.

JOANNE SILBERNER: Call it the Aunt Ethel(ph) syndrome. That's your 85-year-old aunt who smoked for 70 years with no sign of lung cancer yet. According to the American Cancer Society, about eight to 15 percent of smokers will develop lung cancer over their lifetimes.

Jian-Min Yuan of the University of Minnesota and a group of researchers in Asia wanted to know more about this phenomenon.

Dr. JIAN-MIN YUAN (Associate Professor, University of Minnesota): We tried to answer the question, can we use some (unintelligible) marker to predict a risk of lung cancer for people who smoke?

SILBERNER: They had a massive database to work with, years of health information on more than 50,000 people in Singapore and Shanghai. They picked out about 500 who developed lung cancer. Yuan and his colleagues checked urine samples of the afflicted unafflicted smokers for the presence of a particular chemical called NNAL. They came up with a clear answer.

Dr. YUAN: We can predict certain smokers have a higher risk for the lung cancer if they continue to smoke.

SILBERNER: The one-third of smokers with the highest levels of NNAL were twice as likely to get lung cancer as the one-third of smokers in the lowest level. And when the researchers looked for a nicotine byproduct in the urine, the difference was even more dramatic. Those who had the highest levels of both chemicals were at more than eight times the risk of developing lung cancer as people with the lowest levels.

Scientists know that NNAL causes lung cancer in animals.

Tyler Jacks, a lung cancer researcher at MIT, says genetics may dictate how the body deals with various carcinogens in smoke and thus a smoker's risk of lung cancer.

Mr. TYLER JACKS (Researcher, Massachusetts Institute of Technology): The efficiency with which those compounds are broken down will determine how long those harmful compounds are exposed to the cells of the lung or other parts of the body where they could do damage.

SILBERNER: A low reading in Yuan's test could be useful information.

Dr. YUAN: You tell the people you have a lower level of that marker, and maybe you have a relatively lower lung cancer risk, you know, compared to other smokers.

SILBERNER: But ask any cancer specialist whether a negative test for NNAL is good news, and you'll get the answer Jacks gives.

Mr. JACKS: In no way would we suggest that a test of this sort would allow an individual to smoke with no risk at all.

SILBERNER: First off, people with low levels of NNAL still are at some risk of lung cancer. There are plenty of other carcinogens in tobacco smoke. And cigarettes also cause heart disease, emphysema and cancer in other parts of the body.

Could you use a high test result to scare people?

(Soundbite of laughter)

Mr. JACKS: That would be one useful thing to do. If you could identify individuals whose metabolic makeup were such that they were highly prone to lung cancer following cigarette smoking, you might well be able to scare the pants off them.

SILBERNER: And a high NNAL urine test might justify more frequent screening for lung cancer so it can be caught earlier in a more treatable stage, though that remains to be proven.

The results of the study were presented today at the annual meeting of the American Association for Cancer Research.

Joanne Silberner, NPR News.

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