Millions of people worldwide suffer from untreated depression or bipolar disorder, and physicians can struggle to properly diagnose the conditions and assess treatments. Now, a simple blood test may hold the key to unlocking these mysteries.

A protein in our blood called brain-derived neurotrophic factor, or BDNF, plays a vital role in learning, memory and maintaining brain flexibility. Those diagnosed with depression or bipolar disorder have significantly less of this protein than other people. Researchers say that’s because people with these mental health conditions experience increased levels of psychological stress, which reduces the levels of a mature form of this protein, known as mBDNF.

Until now, it has been difficult for researchers to measure levels of mBDNF in blood tests. But a team at the University of South Australia in Adelaide has developed a new blood test that tracks antibodies to identify if these specific proteins are present.

In trials, they were able to detect significantly lower levels of mBDNF in those who had been diagnosed with major depression or bipolar disorder compared with those in a control group. The findings were reported in the Journal of Psychiatric Research.

The study yielded some other interesting results. For example, those taking antidepressants had higher levels of mBDNF in their blood than those who did not take the medication.

A simple blood test, along with a clinical assessment, may soon help physicians and patients get a handle on these debilitating mental health conditions. The next step, the researchers say, is to find ways to increase the levels of these key proteins.