Scientists say they have developed the first blood test to diagnose major depression in teenagers. The test is also able to differentiate between at least two subtypes of depression: major depression and major depression combined with anxiety disorder.

Currently, doctors and psychiatrists diagnose depression solely by relying on the patient to recount his or her own symptoms. Then, the doctor must interpret the patient’s descriptions to decide whether or not the patient is depressed.

However, Northwestern University researchers say a new blood test is able to identify certain ‘gene expression markers’ that can objectively diagnose the depression in teenagers.

“For someone who is depressed, basically every aspect of his or her cognition, perception, mood and ability to interact socially is somewhat impaired,” lead researcher Dr. Eva Redei, a professor of psychiatry and behavioral sciences at Northwestern University Feinberg School of Medicine, told FoxNews.com. “At the moment, diagnosis relies on a structured interview between the doctor and patient - which is not necessarily an objective description of what’s happening.”

Redei said she doesn’t expect the test will eliminate the doctor/patient interviews, but rather, act as a supplement to them.

“[This test] is not to replace psychiatrists’ interviews or their knowledge in that regard, just like a blood glucose test to diagnose diabetes doesn’t replace an endocrinologist,” Redei added. “It merely helps to inform. The idea is to give the same chance to depressed patients – of which there are many – which we are giving to other patients for hypertension, diabetes, etc.”

The test, developed over a period of more than 10 years, first identified 26 potential markers (called messenger RNA) for depression based on animal studies of severely depressed and anxious rats.

Further research in humans found that 11 of those markers could be used to distinguish between depressed and non-depressed patients. The levels of the 11 markers were typically below average range in the depressed patients.

In addition, another set of 12 markers, combined with six of the depression markers, identified the patients who suffered from anxiety as well as depression.

In the most recent study, Redei and her colleagues tested the blood analysis on 28 participants, 14 of whom had been previously diagnosed with major depression and 14 who were not depressed.

Redei’s co-author Dr. Brian Andrus, who was blind to the participants’ diagnoses, was able to use the blood test by itself to accurately determine which participants had depression and which did not. Furthermore, in the pool of depressed patients, Andrus was also able to make the distinction between those who also suffered from anxiety and those who did not.

The next steps are to extend the study to wider patient populations and determine if the test can accurately diagnose depression in adults as well, Redei said. However, she emphasized the long-term goal is not only to provide doctors with a tool to objectively diagnose patients, but also to remove the stigma of depression and better personalize treatment.

Currently, Redei said, there is some shame associated with being diagnosed with depression because patients can feel as if their inability to improve their mood is their own fault. However, she hopes with a blood test proving that depression is in part rooted in genetics, outside of a patient’s own control, the stigma of the disease can be lifted.

“One of the major goals of identifying biomarkers for depression is once you can measure it in a similar way as you would diabetes, you are at least partially removing the stigma, because it’s very difficult to say something like, ‘Get yourself together and control your liver function,” Redei said. “It’s something you can objectively detect and follow and bring into the realm of medicine.”

Once depression is brought into this objective realm of medicine, then it is also easier to distinguish between more subtypes of the disease and treat them accordingly.

“In the future, we could be able to define depression much more profoundly and clearly with a blood test,” Redei said. “It could explain why some drugs work for some patients and not for others. It’s also could help develop new types of antidepressants. Right now, we are guessing. Even the best psychiatrists can’t do anything but prescribe one to three different types of drugs or treatments based on prior experience and trial-and-error.”

Part of the complexity involved in treating depression is that it can manifest in many different ways, according to Redei. Current criteria dictates a person can have a depressed mood in combination with nine different symptoms – but only four or five of the those symptoms need to be present, in any combination, for a patient to qualify as depressed.

“That immediately tells you there’s a large number of potential variations [of depression],” Redei said. “The future goal is to find out whether there are more subtypes of depression, whether we can diagnose them, and whether we can develop treatment paradigms that work better [for those subtypes].”

“The future is really personalized medicine for every disease, and depression is not an exception,” she added.