A blood test can tell patients if they’ve won their battle with cancer.

That’s according to a study by doctors in Australia and at Johns Hopkins University in the U.S., who used a technique called a liquid biopsy to test the blood of 230 people undergoing surgery for colon cancer and found they could predict which ones were likely to see their tumors return.

The results, published today journal Science Translational Medicine, demonstrate that liquid biopsies can categorize cancer survivors as very likely cured or at a big risk for a relapse.

In a traditional biopsy, a chunk of tumor is taken to a lab for study. By contrast, a liquid biopsy uses gene-sequencing machines to scan a person’s blood for tiny fragments of mutated DNA released by dying tumor cells, so it can spot cancers that are otherwise invisible to medical scanners. That has some scientists predicting that liquid biopsies could become a universal screening test to catch cancers early, when they’re most curable.

Investors aren’t far behind the science. Earlier this year, a startup called Grail that is pursuing a doctor’s office screening test able to spot any cancer raised $100 million. That company is led by Jeff Huber, a former Google executive whose wife died of colon cancer.

But until now, what’s been missing from liquid biopsy research is evidence from large controlled studies that the information the test provides is both accurate and useful.

That’s why oncologist Peter Gibbs of the Walter and Eliza Hall Institute of Medical Research in Victoria, Australia, and Bert Vogelstein of Johns Hopkins set out to clarify whether liquid biopsies could help in a very specific situation facing patients with stage II colon cancer.

These cancers are fairly advanced but haven’t yet metastasized, so surgeons can cure 80 percent of patients by cutting the tumors out. But bits of cancer can be left behind, which means some patients will see their tumors come back. Before liquid biopsies, there was no very accurate tool to determine who was cured and who would benefit from an extra course of chemotherapy to mop up remaining cancer cells.

The U.S. and Australian team set out to see if liquid biopsies could make that prediction. They repeatedly took blood from colon cancer patients as they underwent treatment and then tracked them for around two years. Neither the patients nor their doctors were told of the test results.

The researchers found that 79 percent of patients whose blood still harbored tumor DNA after surgery eventually relapsed, while only 9.8 percent of those who tested negative did. That means doctors could be able to use the tests to determine who urgently needs further treatment. At the same time, a negative test would reassure a patient that they have a nine-in-10 chance of being cured, something Tycho Peterson, an analyst at JP Morgan, has called “quantitative peace of mind.” 

Such information will be increasingly valuable, Peterson predicted in a market study published last September by the bank. The report said liquid biopsies of all kinds could balloon into a $22-billion-a-year market by 2020, with “prognostic” tests like the one described today accounting for about a quarter of the industry.

(Read more: “10 Breakthrough Technologies: Liquid Biopsy,” “Spotting Cancer in a Vial of Blood,” “Illumina’s Bid to Beat Cancer with DNA Tests”)