A patient at Thomas Jefferson University Hospital in Philadelphia has blood drawn for a liquid biopsy last year. The test detected a recurrence of breast cancer. (Jacqueline Larma/AP)

CHICAGO — The biggest study to date on new blood tests to detect and analyze cancerous tumors concluded that such "liquid biopsies" are a reliable alternative to conventional biopsies, offering a potentially cheaper and less invasive way of monitoring malignancies.

The study, which involved genetic information from more than 15,000 patients and 50 tumor types, compared tumor samples from liquid biopsies with those from traditional biopsies. In the vast majority of cases, the genetic changes detected by the blood tests agreed with the mutations identified in the tissue biopsies.

"The findings suggest that liquid biopsies provide an accurate snapshot of the genomic landscape of the tumor," said a news release that accompanied the data, which was released Saturday at the annual meeting of the American Society of Clinical Oncology here.

The liquid biopsy test used in the study was Guardant360, which looks for almost 70 mutations. It is made by Guardant Health, which also funded the effort.

Liquid biopsies are a hot field, with several companies, including Foundation Medicine and Grail, either marketing tests or working to develop them. Such tests are designed to pick up in the bloodstream small pieces of DNA shed by cancerous tumors — information that then can be used to treat and monitor the disease.

Currently, doctors largely use surgical biopsies for information about a tumor's genetic mutations and whether the cancer can be treated with available drugs. But such biopsies are invasive and can be expensive and painful. In addition, patients aren't always healthy enough to undergo them, and sometimes not enough tissue is procured to allow pathologists to conduct high-level analysis.

Philip Mack, director of molecular pharmacology at the University of California at Davis's Comprehensive Cancer Center, who presented the findings, said the study indicates that a liquid biopsy can be a "highly informative, minimally invasive alternative" to a traditional biopsy, especially when the cancer is in a difficult-to-reach location such as the brain.

He also said at a news briefing that two-thirds of the results from liquid biopsies in the study were "actionable," meaning patients had mutations that could be treated by drugs.

In the study, the test results were compared with conventional biopsies in two ways. For 398 patients, DNA circulating in the bloodstream was compared with tissue samples previously removed from patients. For the rest of the patients, researchers compared blood test results to those for tissue biopsies in data from large databases.

Mack and other physicians said that traditional biopsies will remain the "gold standard" because they provide more information on the characteristics of tumors and how they are likely to develop. Mack said a trained pathologist can determine what kind of tumor is present based on the appearance of cells in a tissue sample — and whether cancer a extracted from the liver actually originated somewhere else, like in the lung.

But he added that one of the benefits of the liquid biopsy is that it can be done at lower cost and with less effort repeatedly to monitor changes in a tumor over time.

One area in which the two types of biopsies didn't agree as much involved mutations indicating resistance to therapies. Those were picked up in the blood tests but not in the tissue samples — perhaps because those changes occurred after the standard biopsies were completed and treatment was initiated.

The results were welcomed by David Nanus, chief of hematology and medical oncology at Weill Cornell Medicine and New York-Presbyterian Hospital. "To be able to draw that information from a blood test is a huge step forward," said Nanus, who wasn't involved with the study.

He echoed Mack's enthusiasm about tracking a tumor's changes through liquid biopsies. If a patient had a biopsy for colon cancer, for example, and relapsed a few years later, another biopsy would be needed to see how the tumor had changed, he said. A liquid biopsy, he added, could be easier and more convenient.