Cancer Research U.K.-funded scientists have developed a blood test that could help pair cancer patients with the most suitable therapy for their disease and then track the tumor’s progress to see if the treatment is working, according to research published in Clinical Cancer Research.

Using the blood test throughout a patient’s treatment gives a “running commentary” of what is happening to tumors—giving scientists the lowdown on how well the treatment is working, how the cancer is changing and whether it is becoming resistant to treatment. It is the first time a blood test has been used in this way during clinical trials of targeted drugs, proving that the technique can monitor cancer simply and quickly.

The scientists and clinicians, from The Institute of Cancer Research, London, and The Royal Marsden in London, looked at almost 160 blood samples from 39 cancer patients with different types of late-stage cancer.

The test filters out tumor DNA from a patient’s blood to be analyzed for genetic faults. Based on the results, researchers can match the faults to targeted cancer treatments, which then home in on cancer cells carrying these mistakes.

Tumor samples, known as biopsies, are usually only taken at the beginning of treatment, meaning that doctors may be using out-of-date information about how the genetic makeup of a patient’s disease is changing in response to treatment. But the approach could provide real-time updates, as well as helping doctors identify patients who are suitable for clinical trials of new drugs.