Until now, doctors relied solely on a patient's self-described symptoms to diagnose depression. But a new blood test may offer a better tool for quickly identifying and diagnosing this condition.

Sadness, difficulty sleeping, difficulty eating, a general feeling of malaise about life, loss of energy ... these are some of the most common symptoms of depression. But often a patient doesn't put all these symptoms together to understand that depression is the underlying cause. That's when diagnosing depression requires the skills of a health care provider who has time to ask questions, listen to responses and take symptoms seriously.

This new blood test could streamline the process of diagnosing depression and help doctors help their patients more efficiently. The test, revealed in a new study published in Translational Psychiatry was developed with funding from the U.S. National Institute of Mental Health and the Davee Foundation. The test measures the levels of nine genetic markers in the blood. In the study, the blood test was used on 64 adults between the ages of 21 and 79 — half of whom were diagnosed with depression, while the other half were not.  

After the initial blood test, the patients who had been diagnosed with depression underwent 18 weeks of face-to-face or over-the-phone counseling. The test was repeated on 22 of these depressed patients.

Researchers found that the genetic markers — or RNA levels — differed between the patients who were depressed and those who were not. For the depressed patients who underwent therapy, those who self-identified themselves as feeling "recovered" after therapy also showed recovered RNA levels, similar to those of the nondepressed participants. The depressed patients who still felt depressed after therapy had unchanged RNA levels, similar to their initial results. Researchers also noted that — at least in this sample of participants — when five of the RNA markers lined up together the patients were more likely to respond well to therapy. 

The accuracy of the blood test was about 72 to 80 percent — which is similar to the effectiveness of the current interview-style method of diagnosing depression. But health experts are excited about the new development as it marks the first time that a scientific test has been used to confirm the depression diagnosis and predict a patient's potential responsiveness to therapy.

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Could a blood test be used to diagnose depression?
Doctors may be able to use the test to diagnose depression and predict a patient's responsiveness to therapy.