Scientists have developed a blood test that can diagnose depression in teens and young adults.

Currently psychiatrists can only diagnose depression by relying on a patient’s willingness to report symptoms and their ability to interpret them, and diagnosis for teenagers has typically been more difficult because of the normal emotional ups and downs of adolescence. 

Researchers at Northwestern University said that the new blood test can identify certain “gene expression markers” that can objectively diagnose depression.

"Right now depression is treated with a blunt instrument," researcher Eva Redei, a professor of psychiatry and behavioral medicine at the Northwestern Feinberg School of Medicine in Chicago said in a statement. "It's like treating type 1 diabetes and type 2 diabetes exactly the same way. We need to do better for these kids."

Researchers from the study, published on Tuesday in the journal Translational Psychiatry, were able to identify 26 markers of major depression in rats that had genetic and environmental predispositions for depression. 

Afterwards, researchers looked for the same depression markers in the blood of 28 participants between the ages of 15 to 19, 14 with depression and the other without, and found that 11 of the markers found in rats were also found in depressed teens but not in teens without the mental disorder.

"These 11 genes are probably the tip of the iceberg because depression is a complex illness," Redei said. "But it's an entree into a much bigger phenomenon that has to be explored. It clearly indicates we can diagnose from blood and create a blood diagnosis test for depression." 

Researchers said that the blood can also identify subtypes of depression, like patients with a combination of depression and anxiety disorder. 

"This is the first significant step for us to understand which treatment will be most effective for an individual patient," Redei said. "Without an objective diagnosis, it's very difficult to make that assessment. The early diagnosis and specific classification of early major depression could lead to a larger repertoire of more effective treatments and enhanced individualized care."

Health experts say that it is important for depression to be detected early in life, especially in teens because if the disorder puts teens at a greater risk for other health hazards like substance abuse, physical illness and suicide. Researchers added that when depression begins earlier in life, there is also a greater possibility that it will persist and worsen in adulthood.

The National Institute of Mental Health estimates that 11 percent of adolescents aged 13 to 18 develop major depression at some point in their teenage years, and Substance Abuse and Mental Health Services Administration reported that in 2008 8.3 percent of children aged 12 to 17 were depressed.

Researchers at Massachusetts General Hospital had also reported in February in the journal Molecular Psychiatry  that they had found levels of nine biomarkers in the blood of adults with major depressive disorder that differed from health patients.

While the latest study is not the first to develop a blood test for major depression, Redei said that her test is different because it identified blood markers that had not previously been linked to depression. 

Researchers say that it is unlikely that the blood test would replace interviews with doctors, but be more as a supplementary tool to help diagnose it. 

Some experts are still skeptical about how useful a blood test can be in identifying and treating depression. 

"A blood test doesn't show the dynamics of what the particular person is dealing with, where their depression is coming from. You've got to explore it at a much deeper level than just a blood test," Elaine Leader, a psychiatrist and the executive director of TEEN LINE, a mental health hotline for California teens, told ABC news.

Other psychiatrists say that biologic testing doesn’t account for the complex interactions of genetic, biological and environmental factors that all are involved in depression. 

"I think people are looking for a magic bullet, a single answer. But these disorders are much too complicated," Dr. Carol Bernstein, an associate professor of psychiatry at New York University Langone Medical Center told ABC. "But certainly the more tools we have the better."

Redei said that the blood test is not just important for diagnosing patients, but also for minimizing the stigma attached to the disorder.

"Everybody, including parents, are wary of treatment, and there remains a social stigma around depression, which in the peer-pressured world of teenagers is even more devastating," Redei said. "Once you can objectively diagnose depression as you would hypertension or diabetes, the stigma will likely disappear."

Researchers are not testing how well the blood test works on adults.