Doctors may have found a new way to detect lung cancer during its earliest stages using a simple blood test, according to research scheduled to be presented in Toronto this week.

Investigated by doctors at the University of Pennsylvania, the non-invasive test would be the first of its kind for distinguishing between cancerous and benign lung lesions, which are traditionally screened for using CT scans.

Detailed CT scans can detect a range of abnormalities, including scars or small areas of infection, but have a high false-positive rate that requires many patients to undergo extensive follow-up procedures like serial CT scans, PET scans or biopsies — even when only a small percentage of problems turn out to be cancer-related.

"So this is a simple blood test in which [doctors] look at certain genes to see whether they can distinguish the people that coulddhave lung cancer versus the people who don't," said Dr. Peter Ellis, an associate professor in the oncology department at McMaster University in Hamilton, Ont.

The new test was developed by taking blood from 44 people with early-stage lung cancer, as well as from 52 others who had no cancer at all. Researchers examined the blood for white blood cells that help regulate the immune system, and their relation to a large number of genes.

Doctors found that if they looked at 15 particular genes, they were able to distinguish the people who might have lung cancer from those who didn't with about 87-per-cent accuracy.

While a biopsy would be required to make a conclusive diagnosis, Ellis said the new test could help make early distinctions between "people who clearly don't have cancer and people who are very likely to have cancer.

"If a simple blood test can distinguish between people who don't have cancer, then that really is, I think, a significant advance."

The research will be presented by its American authors at the American Thoracic Society's 2009 International Conference in Toronto on Tuesday. Ellis said the test, as promising as it appears, is still a few years in the making before it could be available to the public.

A similar blood test called a PSA is used to detect prostate cancer.