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British scientists took a major step toward bringing personalized medicine to mental health on Tuesday, announcing that they developed a blood test that can accurately predict whether you'll respond to the conventional, commonly prescribed antidepressants on the market.

This is important because about half of patients who try them don't respond to first-line antidepressants and often spend months struggling to find the right drug. Doctors also have not had a lot of tools to help them create more personalized individual treatment plans.

The new test, described in the International Journal of Neuropsychopharmacology, measures levels of two biomarkers known as macrophage migration inhibitory factor (MIF) and interleukin 1 beta that indicate inflammation. Researchers have previously shown that inflammation is associated with poor response to some of the more commonly prescribed antidepressants. In this study, researchers were able to quantify that effect by pinpointing the threshold at which these medicines stop working. Above the threshold, the study found that patients had a 100 percent chance of not responding. They said that those with readings below the threshold could be "expected" to respond to those same treatments.

Carmine Pariante and her colleagues at King's College London suggested that "guided by this test, patients with blood inflammation above a certain threshold could be directed towards earlier access to more assertive antidepressant strategies, such as a combination of antidepressants, before their condition worsens."

The researchers said that this is the first time a blood test has been shown in the published peer-reviewed literature to "precisely predict" a patient's response to antidepressants. But before drawing any definitive conclusions, they said, more work is needed to compare the current clinical practice in prescribing drugs with an approach using their test.

Meanwhile, the era of personalized medicine in psychiatry has already begun in the United States. Some doctors have begun using a saliva-based DNA test sold by Pennsylvania-based Genomind to help them choose the right drugs for patients. Genomind says its product covers 15 drug classes for psychiatric drugs and 103 medications that treat a wide variety of conditions. The test looks at genetic encoding that makes some people slow or fast metabolizers of certain drugs — which can result in a range of consequences from the medicine not working to severe side effects — as well as for variations in the serotonin transporter, the site where many commonly used antidepressants work.

Genomind has said its test is not subject to U.S. Food and Drug Administration approval, and only some insurers will pay for it on a case-by-case basis.

There are a number of other ongoing studies looking at other biomarkers that may predict a patient's response to antidepressants. In a study published in Nature Medicine in 2014, for example, researchers looked at the role of small molecules known as miRNAs in the brain. They found that people who committed suicide had significantly lower levels and that antidepressants appear to change the levels of this microRNA.

"We found that miR-1202 is different in individuals with depression and particularly, among those patients who eventually will respond to antidepressant treatment," Gustavo Turecki, a researcher in the McGill University psychiatry department, said at the time in a news release.

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