A simple blood test could change how doctors decide who needs chemotherapy, and how intensive it should be, in patients with colon or bladder cancer, according to study results presented at the ESMO meeting.

More than 1,000 people with stage 3 colon cancer in one study had blood taken about six weeks after surgery to remove the tumor.

If tiny fragments of cancer DNA in the bloodstream, known as circulating tumor DNA, were not detected, patients were considered ‘low-risk.’ If ctDNA was present, they were deemed ‘high-risk.’

The intensity of their treatment was then guided by their ctDNA results, according to a report of the study published in Nature Medicine.

Low-risk patients were able to receive less chemotherapy, leading to fewer hospitalizations and a reduction in side-effects such as nerve damage, with only slightly lower cancer-free survival, the researchers said in a statement.

“Outcomes were excellent among the patients identified as low risk based on their ctDNA levels, with 87% remaining cancer-free three years after surgery,” the researchers said.

A separate international trial published in The New England Journal of Medicine found that blood tests for ctDNA can also help guide treatment with Roche's immunotherapy Tecentriq (atezolizumab) in patients who have had surgery to remove bladder tumors that invaded the muscle.

“By selecting patients based on ctDNA, we see a benefit not only for disease-free survival but also for overall survival with (Tecentriq),” study leader Dr. Joaquim Bellmunt of Dana Farber said in a statement.

“This is the first time that an adjuvant immunotherapy trial has shown a benefit for survival in selected patients based on ctDNA testing.”