A blood test that looks for the body’s immune response to tumors may provide an easy way to detect lung cancer in patients long before an X-ray or CT scan can, U.S. researchers reported on Friday.

The test correctly predicted non-small-cell lung cancer, the most common type of lung cancer, in blood samples taken from patients years before they were actually diagnosed with lung cancer, the researchers reported.

If the test’s reliability can be confirmed, it might become the first new blood screen for any cancer since the prostate-specific antigen (PSA) test. The test is licensed to privately held Rockville, Maryland-based 20/20 GeneSystems Inc.

“These data suggest antibody profiling could be a powerful tool for early detection when incorporated into a comprehensive screening strategy,” the researchers wrote in their report, published in the Journal of Thoracic Oncology.

Non-small-cell lung cancer is the most common type of lung cancer, and has an average 5-year survival rate of only 40 percent.

Lung cancer is by far the biggest cancer killer globally. Each year 10 million people are diagnosed with it, according to the Global Lung Cancer Coalition, and half of all patients die within 1 year of diagnosis. It kills more than 160,000 people annually in the United States alone.

Computed tomography imaging test, also known as CT scans, can find lung cancer tumors, but there is a high rate of false positives - meaning many people undergo a biopsy to remove a piece of a suspicious lump from the lung, only to find out it is not cancerous after all.

By the time people have symptoms of lung cancer, it is usually too late to cure them.

Li Zhong and colleagues at the University of Kentucky developed a test that looks for certain proteins the body makes in response to very early lung tumors.

When they tested it in people who were being treated for lung cancer, it correctly identified 90 percent of cases, with very few false positive results.

They went back and tested blood samples from some of the lung cancer patients years before they were diagnosed. The test found cancer in four out of seven samples taken 1 year before diagnoses, and in all 18 samples taken 2, 3 and 4 years earlier.

“Based on doubling times, a lung cancer can be present 3 to 5 years before reaching the conventional size limits of radiographic detection,” Zhong’s team wrote.