If the test continues to perform well in studies, experts say it could become one of the first objective ways to look for depression, which affects nearly 1 in 10 American adults.
"Psychiatry is a field that is begging for tests because all of our diagnoses, for the most part, are based on clinical assessments, and clinical assessments are very subjective and can be biased," says Jennifer L. Payne, MD, a psychiatrist and co-director of the Mood Disorders Center at Johns Hopkins Hospital in Baltimore.
Payne reviewed the study for WebMD but was not involved in the research, which is published in the journal Molecular Psychiatry.
"We'd love to have tests that tell us if someone has a particular illness or not," she says. "This is a very nice step in that direction, but there's a heck of a lot more work to be done before this is used as a clinical test for major depression."
Study Results 'Quite Exciting'
The new test, called MDDScore, was developed by a company called Ridge Diagnostics. And it's not the only objective test for depression under study.
In 2010, the company Rules-Based Medicine, which is headquartered in Austin, Texas, began offering a blood test that checks for recent-onset schizophrenia. The company is reportedly tweaking that test to make a version that looks for depression.
MDDScore measures nine indicators in blood called biomarkers that are thought to be altered in adults with major depressive disorder.
"A large number of the markers that constitute the test come from the inflammation family," says researcher George Papakostas, MD, a psychiatrist and director of treatment-resistant depression studies at Massachusetts General Hospital.
"Chronic inflammation [is] part of a risk factor or part of the process of depression itself," says Papakostas, who is also an associate professor of psychiatry at Harvard Medical School and a consultant for Ridge Diagnostics.
Additional indicators include hormones, growth factors, enzymes, and other proteins that act as chemical messengers.
In two studies that involved 70 people with depression and 43 people who were not depressed for comparison, the test correctly identified depression about 91% of the time and correctly ruled it out about 81% of the time.
"Those numbers are high, and I think that's quite exciting," says Payne.
Still, the test missed depression in about 9% of people tested and falsely diagnosed depression in about 19% of people who were actually healthy.
Papakostas says those error rates may be acceptable since the test isn't meant to be used as a standalone diagnostic tool.
"The goal in developing a test is not to kind of nullify clinical judgment or patient experience, but to kind of supplement or help with that," he says.
According to the Ridge Diagnostics' web site, the test is available to patients but must be ordered by a doctor. The list price of the MDDScore is $745. The cost to patients is $90 if it is covered by insurance.
A Depression Test Is Available, but Should You Get It?
The studies have a number of limitations. They were small, which makes their results less generalizable to a wide range of people.
For example, women were underrepresented in both groups that were used to assess the test.
Women are diagnosed with depression about twice as often as men, yet they made up just over one-third and one-half of the depressed patients who were tested.
And the comparison group of healthy adults, which should ideally mirror the groups of depressed adults in terms of age and sex, was much different. The healthy group was younger, had significantly more women, and weighed less, on average, than the people who were depressed.
That could make any comparisons flawed.
And Papakostas acknowledges that there are many critical unknowns that still need to be figured out before the test should be used in clinical practice.
It's not clear, for example, if the test can distinguish patients who have depression from patients who have bipolar disorder. Patients who are bipolar experience euphoric mood swings in addition to episodes of depression, and the treatment for the two conditions is very different.
The next step, Papakostas says, is to study the test as a screening tool on a wide variety of people with unknown conditions and see how it stands up to more traditional diagnostic tools like questionnaires and clinical assessments of symptoms.
"Can this differentiate between depression and normal grief? Can this track treatment outcomes? Can this guide research? We have to find out," he says.
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