Researchers at Northwestern University in Chicago have developed a blood test that may be able to identify people suffering from clinical depression.

“Treatment starts with a precise diagnosis,” said Eva Redei, the paper’s lead author and a professor of psychiatry and behavioural sciences at Northwestern.

Doctors currently diagnose the disease by looking at symptoms — like identifying an animal by examining the size and shape of its paw prints. With an accurate, objective blood test, doctors could get a look at the animal itself.

Researchers at Northwestern took blood samples from 32 patients with clinical depression, as well as 32 samples from patients without depression to use as a control.

Their work was published Tuesday in Translational Psychiatry.

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Samples taken from patients with depression showed different levels of nine markers than patients who were not depressed.

Markers are chemicals which can indicate the expression of a particular gene. Testing for the identified markers could allow doctors to find out if a patient is clinically depressed, Redei said.

“It’s an objective test, so this will not depend on whether a patient wants to talk or is able to talk to a physician,” she explained.

The test also appears to predict whether a patient with depression will respond to cognitive behavioural therapy.

Samples taken before therapy showed that patients who improved after therapy had different levels of several markers than patients who did not.

Both groups of patients had similar levels of three other markers, which the researchers said could point to a basic genetic predisposition for major depression.

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The research could also guide scientists looking for similar tests to diagnose other mental illnesses, like post-traumatic stress or anxiety disorders, Redei said.

In 2012, about 5.4 per cent of Canadians over age 15 reported that they had experienced symptoms of major depression or mood disorders, according to Statistics Canada.

Redei said the researchers hope to confirm their results in larger studies.

If they are able to find funding, a larger study could be finished in about two years, she said.

Depending on those results, researchers could then submit a test to the U.S. Food and Drug Administration for approval. It could then be ordered and administered like any other blood test.