Blood has many roles in the metaphorical life of the human body. It’s the organ of kinship (“blood ties”), the seat of emotion (“blood lust”) and a vehicle for identity (“travel is in his blood”). But might it also be a window on the body’s fate? Many scientists are trying to answer that question as a practical matter, not a metaphor. They are scouring the blood for biomarkers — easily measured substances that illuminate what’s going on in hard-to-reach places.

Biomarkers aren’t new to medicine. Many lab tests are, in fact, tests of biomarkers. Sugar or protein in a sample of urine can shed light on what’s happening in the kidneys or pancreas. The concentration of cholesterol in the blood may hint at disease in the arteries. What’s changed is medicine’s ability to measure more molecules, with greater precision and less cost, than was possible in the past. READ MORE