U.S. researchers have developed a blood test that can diagnose depression in adults by identifying nine different markers that are linked to the illness.

The blood test provides doctors with the first objective method of scientifically diagnosing depression, according to researchers at Northwestern University.

A study describing how the test was developed was published Tuesday in the journal Translational Psychiatry.

The research group said the test can also predict who is vulnerable to depression and which patients will benefit from cognitive behavioural therapy. The blood test can also show whether therapy is effective for a particular patient, by measuring the changes in the levels of the different markers in the blood.

Prof. Eva Redei, who developed the test and co-authored the study, said it marks a major medical breakthrough. Redei had previously developed a blood test that diagnoses depression in teens.

"This clearly indicates that you can have a blood-based laboratory test for depression, providing a scientific diagnosis in the same way someone is diagnosed with high blood pressure or high cholesterol," she said in a statement. "This test brings mental health diagnosis into the 21st century and offers the first personalized medicine approach to people suffering from depression."

The test works by measuring the levels of nine specific RNA blood markers. RNA (ribonucleic acid) is a group of molecules that perform different roles in interpreting the DNA genetic code.

In search of a biological test

Co-lead author Prof. David Mohr said that scientists have been searching for years for a biological diagnostic test for depression.

The current method of diagnosing depression is based on reporting and assessing non-specific symptoms including poor mood, fatigue and changes in appetite.

A diagnosis is based on the patient's ability to report their own symptoms, as well as the doctor's ability to interpret them. This can be problematic, as many depressed patients underreport their symptoms or can't adequately describe them, the researchers said.

"Mental health has been where medicine was 100 years ago when physicians diagnosed illnesses or disorders based on symptoms," Mohr said in the statement. "This study brings us much closer to having laboratory tests that can be used in diagnosis and treatment selection."

To develop the blood test the researchers tested 32 patients between the ages of 21 to 79 who had been previously diagnosed as depressed in a clinical interview. They also tested 32 non-depressed controls in the same age range.

The researchers found nine different RNA blood markers with levels significantly different in the depressed patients from those of the controls. It is these same nine markers that can diagnose depression, according to the study.

The patients then received 18 weeks of therapy and were assessed again through clinical interviews to see if they were still depressed. Their blood was also tested again to see if there was any change in the levels of the markers.

Scientists observed that the levels of certain markers could differentiate between those patients who had responded positively to the therapy and those patients who remained depressed. They said this marks the first biological indicator of successful cognitive behavioural therapy.

Additionally, the test also appears to indicate who may be vulnerable to depression.

Redei found that three of the nine RNA markers remained different in depressed patients compared to the controls, even if the depressed patients improved after therapy.

"These three markers move us toward the ultimate goal of identifying predisposition to depression, even in the absence of a current depressive episode," she said in the statement.

In the future, Redei said she plans to test the results of the small study in a larger group. She would also like to see if the test can differentiate between depression and bipolar disorder.