With a drop of blood and some laboratory analyses, doctors have been able to tell patients whether they suffer from diabetes or some sort of cancer. Measuring changes in the cells or molecules of human tissues or fluids, by detecting so-called biomarkers, has aided the diagnosis of various diseases for some time. But when it comes to severe mental illnesses, such as schizophrenia, "patients are just diagnosed by asking questions," says Sabine Bahn, head of the Cambridge Centre for Neuropsychiatric Research, at the University of Cambridge, UK. "This hasn't changed over the last 100 years.

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