A patient suspected of having cancer usually undergoes imaging and a biopsy. Samples of the tumor are excised, examined under a microscope and, often, analyzed to pinpoint the genetic mutations responsible for the malignancy. Together, this information helps to determine the type of cancer, how advanced it is and how best to treat it. Yet sometimes biopsies cannot be done, such as when a tumor is hard to reach. Obtaining and analyzing the tissue can also be expensive and slow. And because biopsies are invasive, they may cause infections or other complications.

A tool known as a liquid biopsy—which finds signs of cancer in a simple blood sample—promises to solve those problems and more. A few dozen companies are developing their own technologies, and observers predict that the market for the tests could be worth billions.

The technique typically homes in on circulating-tumor DNA (ctDNA), genetic material that routinely finds its way from cancer cells into the bloodstream. Only recently have advanced technologies made it possible to find, amplify and sequence the DNA rapidly and inexpensively.

Right now, the tests, which are available from several companies, mostly aid in treatment decisions for people already diagnosed with a particular form of cancer, such as prostate or lung. ut the liquid tests can also provide services that tissue biopsies cannot. Repeated tests could potentially detect disease progression or resistance to treatment long before it would trigger symptoms or appear on imaging. Tissue biopsies examine only selected bits of tumors and can thus miss cells that have turned more dangerous than their neighbors; in principle, the liquid biopsy can detect the full spectrum of mutations in a mass and thus indicate when more aggressive treatment is needed. And importantly, liquid biopsies may one day provide a fast, easy screening test for detecting a cancer and determining its type in people who seem perfectly healthy.

In a sign of the growing enthusiasm for the field, in March GRAIL, a company spun off from Illumina, raised $900 million in funding for this last application from investors including Amazon and several major pharmaceutical companies.* GRAIL plans to use the money to further develop the technology and to run the large clinical trials (involving hundreds of thousands of subjects) needed to see if screening will be feasible. Also in March the California-based company Freenome received $65 million for clinical trials, expected to be carried out with multiple research partners, to determine whether the testing improves how cancer patients fare. And this past May Guardant Health announced it had raised $360 million from investors, on top of earlier funding, with the goal of deploying its liquid biopsy test to one million people over the next five years.

For the tests to enter wide usage, clinical trials must prove that the approach detects cancer accurately and that by aiding in treatment decisions, it improves progression and survival rates.

*Editor's Note (6/26/17): This sentence was edited after posting. The original erroneously stated Illumina spun off from GRAIL.