People suffering from bipolar disorder swing between emotional highs (manic episodes) and severe depression. Perhaps not surprisingly, they’re often mistakenly diagnosed as simply having major depression, which actually requires a different treatment. According to a new study from Illinois’ Loyola University, however, the two conditions can be told apart by monitoring the patient’s heartbeat.

When treating a bipolar patient’s depression, an antidepressant is typically prescribed along with a mood stabilizer or antipsychotic drug, the latter keeping the antidepressant from triggering a manic episode. This is why it’s important to be able to differentiate between bipolar disorder and major depression.

To that end, the Loyola team studied 64 adult test subjects with major depression, and 37 with bipolar disorder. All 101 of them initially underwent 15-minute-long electrocardiograms, which were subsequently analyzed using special software.

It was discovered that the subjects with major depression all had a higher respiratory sinus arrhythmia – which is a type of heart rate variability – than those with bipolar disorder. That said, further research is required in order to confirm the findings.

“Having a noninvasive, easy-to-use and affordable test to differentiate between major depression and bipolar disorder would be a major breakthrough in both psychiatric and primary care practices,” says Dr. Angelos Halaris, who led the study.