A recent study from scientists at Northwestern University has resulted in the world\u2019s first blood test to diagnose major depressive disorder in adults and determine who might benefit from cognitive behavioral therapy. The test measures the levels of nine RNA blood markers that were identified during the course of the study. In addition to diagnosing adults who are currently suffering from major depression, the test may also be able to indicate individuals who may develop the disorder in the future. Long Search for Scientific Tool to Diagnose Depression Mental illness presents many challenges for the medical community, and one of the greatest challenges is accurate diagnosis. Correctly identifying mental illness can be extremely difficult for a variety of reasons, including overlapping symptoms and the fact that most of the information medical professionals gather about an individual\u2019s symptoms comes from the patient\u2019s own descriptions, which can be very subjective. As study co-author David Mohr says: \u201cMental health has been where medicine was 100 years ago when physicians diagnosed illnesses or disorders based on symptoms.\u201d This can lead to errors as physicians gather and interpret the symptoms, and significantly delay treatment. There is often a long delay in the diagnosis of the first episode of major depression: one 2013 study found an average of nearly 10 weeks before first episodes of depression were diagnosed. The ability to use biological markers to diagnose mental illness would significantly improve accurate and rapid diagnosis as well as the delivery of effective treatment. The large and rising number of people affected by major depressive disorder\u2014currently around 6.7 percent of the adult population of the United States\u2014has made the search for a scientific test for this disorder a top priority. Identifying Depression and Possible Treatment The study identified nine RNA markers in 32 patients with independent diagnoses of depression that were at significantly different levels than these nine markers in 32 control patients. The 64 individuals in the study were between the ages of 21 and 79. At the baseline of the study, the nine RNA markers were able to diagnose those who were currently depressed. The patients were then given an 18-week course of therapy, after which their RNA markers were evaluated once again. Those whose depression symptoms had lessened with therapeutic treatment showed changes in the levels of these markers, while those whose depression was unchanged showed no changes in these markers. Furthermore, the researchers were able to look at the original levels of the nine markers in the depressed patients and identify baseline patterns in those who ended up benefitting from the therapy. This suggests that physicians could use the test to identify which patients with depression will do well in cognitive behavioral therapy and which patients might be more effectively treated with medication. Certain Markers Suggest Vulnerability to Depression Three of the nine markers used to diagnose depression showed different blood levels in some patients even if their depression had been successfully treated. The researchers believe that these levels indicate a stronger vulnerability to depression, and could be used to identify adults who are at greater risk for developing this mental illness. This could lead to even more rapid diagnosis of this disorder during the first episodes of depression, and could also be used to help prevent the development of depression in individuals at high risk through efforts to promote general wellness and keep stress at manageable levels.