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A fast blood test can help guide doctors to the best drugs for treating lung cancer patients.

The so-called liquid biopsy accurately told whether a lung cancer patient had a mutation that makes the disease treatable with certain pills that can have remarkable effects. The test could also tell if less-fortunate patients had a different mutation, saving them weeks or months of treatment that would ultimately be useless.

It’s a big step in the field of personalized medicine, said Dr. Wafik el-Deiry of the Fox Chase Cancer Center in Philadelphia, who was not involved in the research.

“This study is not about the promise, it’s about realizing the promise,” El-Deiry told NBC News.

Jeannie Larson of Merrimack, New Hampshire is one of the first patients to try the test. The 57-year-old mother of two was stunned when she was diagnosed with stage 4 lung cancer. Like so many lung cancer patients, she hadn’t had clear symptoms.

“I’ve never smoked and lived healthfully, so when I got that diagnosis it was pretty shocking,” Larson told NBC News.

“They said, ‘Oops. your lung has collapsed'."

Doctors had to do a biopsy, go into her lung and get a little piece of the tumor to test it, so they’d know for sure she had lung cancer — and try to figure out what kind of mutation was causing the cancer, so they’d know how to treat it.

These biopsies can be dangerous, and Larson suffered a severe side effect — a collapsed lung.

“They said, ‘Oops, your lung has collapsed. You know, you're going to have to stay here for a while’,” Larson said.

“And so they had to stick a tube in my side and I was connected to some sort of machine that was supposed to inflate it and it took a long time to inflate. The last thing I really ever wanted to do was have to go through it again.”

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Like many nonsmokers, Larson had a mutation in a gene called epidermal growth factor receptor (EGFR). That was good news, because she was eligible for treatment with Tarceva, a drug that specifically targets EGFR mutations.

“I was able to relax quite a bit because it was like, 'OK, we've got this.' Even though I knew it wasn't a permanent cure, but it was still going to extend my life and give me a much better quality of life,” Larson said.

It helped for a while, but the cancer came back.

Her oncologist, Dr. Geoffrey Oxnard of the Dana Farber Cancer Institute in Boston, was working to develop the liquid biopsy at the time. Larson didn’t want to go through another biopsy.

“He said 'yes, we can do a test that can do the same thing as a tissue biopsy. We can do a blood test,' and that was quite a relief,” Larson said.

The test looks for DNA from dead, broken-open tumor cells in the plasma, the liquid part of the blood. Scientists have known for years this free-floating DNA is there, but they have struggled to develop a test that can find it.

This one detects certain mutations in EGFR that affect many, but not most, lung cancer patients and mutations in another gene called KRAS that affect mostly smokers with lung cancer. Several groups, including more than three dozen companies, are working to develop these tests.

Oxnard’s team tried out their test in 180 lung cancer patients.

“There has been skepticism will liquid biopsy always match the tumor. This study is inspiring.”

“This assay exhibited 100 percent positive predictive value for the detection of these mutations,” they wrote in the Journal of the American Medical Association’s JAMA Oncology.

A tissue biopsy takes about four weeks to provide results. This test, they said, brought back results in three days — a big difference for lung cancer patients who want to know if they have even a chance of living a bit longer.

“There has been skepticism, will liquid biopsy always match the tumor?” said El-Deiry. “This study is inspiring.”

Matching treatment to patient

Lung cancer is the leading cause of cancer death in the United States. It will be diagnosed in more than 224,000 people this year and it'll kill nearly 160,000.

It’s very hard to cure, but the right treatments can help patients live much longer without serious symptoms. The trouble has been matching the right treatment to the right patient.

Cancer, it turns out, is not one disease but hundreds, each one caused by a different genetic mutation. What doctors hope to get from personalized medicine is better treatment that doesn't waste anyone's time with drugs that will not help the patient get better.

Right now, Larson is benefiting. The liquid biopsy showed her cancer had developed what’s called resistance to Tarceva, but suggested it might be knocked back by a newer drug called Tagrisso, or osimertinib.

It’s another pill. “And I can't even tell that I'm taking a medication, I really can't,” she said.

“I feel like I take the pill in the morning and that's the last I really think about it.”

For patients with the other mutations, in the KRAS gene, there are choices such as the drug Alimta, or pemetrexed.