CAMBRIDGE, UKA new blood test promises quicker assessment of treatment efficacy in patients with metastatic breast cancer. Dr Dana Tsui and colleagues from the Cancer Research UK Cambridge Institute, University of Cambridge, report in the New England Journal of Medicine that circulating tumour DNA is a more powerful marker of breast cancer progression than using blood tests for cancer cells or antigens, and raises the possibility of screening the effectiveness of drugs in days rather than months needed to wait for a change in radiological appearance.