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A definitive test for depression -- it takes the guesswork out of diagnosis. By doing that, it may help take the stigma away from what has been in our society a difficult diagnosis.

Depression does not discriminate -- it strikes people of all ages, races and income levels.

But figuring out the problem and the potential therapy is very arbitrary without the proper medical team.

Now Northwestern Medicine researchers say they have developed a blood test that removes the questions from a depression diagnosis.

Walking the streets -- you have no idea the inner demons someone is battling ... the sadness behind the smile, the struggle to just get through the day. When hiding becomes the norm, it makes it even more difficult for doctors or therapists to help.

Dr Eva Redei, Northwestern medicine depression researcher: “And at the moment depression is diagnosed by a clinical interview, and if the patient is not able or not willing to describe their symptoms, that diagnosis is obviously not very accurate.”

And that means a major delay in identifying the problem and getting the proper help.

Dr Redei: “It’s not a surprise it is between two and fourteen months of delay from the onset of depression and the diagnosis, and that’s a long, long time.”

Precious time a patient could have sought behavioral therapy or medication to alleviate their symptoms and lift them from their depression -- and time their friends, family, loved ones and acquaintances could truly understand the nature of their illness.

Dr Redei: “We make that separation between medical illness and mental illness. Our brain is part of us. When it is sick, it is just as sick medically sick than your kidney or your heart.”

So researchers at Northwestern Medicine began studying depression in the lab. Instead of looking at questionnaires about how people feel they look at the brain, beginning with a rat brain.

Dr Redei: “We found markers that were the same in the brain and the blood in these animal models. Then we translated them to humans, and most of these markers are also present in human blood.”

Nine markers to be exact -- present in rats and all humans. When the levels fluctuate -- some higher, others lower, it indicates depression.

Dr Redei: “These markers indicate the presence of illness.”

It's similar to cholesterol markers which change depending on whether a person is unhealthy.

Dr Redei: “Just like cholesterol markers, levels of the markers reflect that somebody is currently having a depressive episode.”

The markers also showed people who had a pre-disposition to depression. Then when the test subjects got therapy, those who felt better actually had changes in their blood markers.

Dr Redei: “So it is suggestive that there is something different happening in the patients that respond to cognitive behavioral therapy, which is psychotherapy.”

With no delay a rapid blood test, offering a quick, definitive diagnosis paves the way for a better future for anyone battling depression.

Dr Redei: “The sooner somebody is diagnosed and treated, the higher the probability they will get better faster.”

Doctors are hoping to perform a larger study to confirm their current findings -- and now they want to add another element: medication therapy to see if it produces similar changes in the blood indicating it worked. Another goal is to differentiate between types of depression for example bipolar or post-partum. That would give doctors a better idea of exactly what therapies to prescribe.