In the usual cancer biopsy, a surgeon cuts out a piece of the patient’s tumor, but researchers in labs across the country are now testing a potentially transformative innovation. They call it the liquid biopsy, and it is a blood test that has only recently become feasible with the latest exquisitely sensitive techniques. It is showing promise in finding tiny snippets of cancer DNA in a patient’s blood.

The hope is that a simple blood draw — far less onerous for patients than a traditional biopsy or a CT scan — will enable oncologists to quickly figure out whether a treatment is working and, if it is, to continue monitoring the treatment in case the cancer develops resistance. Failing treatments could be abandoned quickly, sparing patients grueling side effects and allowing doctors to try alternatives.

“This could change forever the way we follow up not only response to treatments but also the emergence of resistance, and down the line could even be used for really early diagnosis,” said Dr. José Baselga, physician in chief and chief medical officer at Memorial Sloan Kettering Cancer Center.

Researchers caution that more evaluations of the test’s accuracy and reliability are needed. So far, there have been only small studies in particular cancers, including lung, colon and blood cancer. But early results are encouraging. A National Cancer Institute study published this month in The Lancet Oncology, involving 126 patients with the most common form of lymphoma, found the test predicted recurrences more than three months before they were noticeable on CT scans. The liquid biopsies also identified patients unlikely to respond to therapy.