Commentary—Can a Blood Test Detect Major Depressive Disorder?

Researchers at the Yale School of Medicine recently published results of a study on a blood test that appears to help tell whether someone has the serious form of depression called major depressive disorder (MDD). You might think this would be an easy thing for doctors to tell, but it’s not. For example, some people with MDD have hallucinations and delusions like people with schizophrenia. And some people with schizophrenia appear depressed. A test that could tell the difference might help doctors when the diagnosis isn’t clear. Also, such a test might be able to predict who is likely to have MDD before they become seriously impaired.

When doctors hear about a new test, they ask several things:

• How does the test work?
• Is the test accurate?
• Even if it’s accurate, will this test help me take care of my patients?

How does the test work?

The test is based on the fact that certain chemical receptors in brain cells function abnormally in people with certain major psychiatric disorders. One of these receptors is also involved in maintaining the body’s balance of water and salts (electrolytes, such as sodium and potassium). Thus, the researchers’ theory was that because the same receptor is involved, people with certain psychiatric disorders might have an abnormal response to changes in their body’s fluid balance.

To test this theory, the researchers gave an IV of concentrated salt solution to 3 groups of patients: One group with schizophrenia, another with major depressive disorder (MDD), and a third group of well people. An increase in sodium in the blood makes a person’s pituitary gland secrete a hormone called antidiuretic hormone (ADH), or vasopressin. They thought that people with psychiatric disorders might secrete different amounts of ADH than well people. So, after giving people the IV salt solution, the researchers measured the level of ADH in their blood several times over 2 hours and compared how the levels rose among the three groups of people.

Is the test accurate?

Somewhat. The researchers did find that people with MDD secreted on average more ADH after getting the salt solution than either people with schizophrenia OR the well people. However, this wasn’t true for every single patient with depression, so the test doesn’t say for sure. Also, there wasn’t any real difference between the people with schizophrenia and the well people, so the test wouldn’t help diagnose schizophrenia.

Doctors will need to repeat this study in new groups of patients to see if the findings in people with depression hold up.

Will the test help doctors take care of patients?

Not just yet. The findings are important scientifically. They help show the abnormal brain chemistry that is present in some people with psychiatric disorders, and the test did identify a number of people who had MDD. However, the test missed some patients, so it isn’t quite accurate enough. And the test requires many hours and multiple blood samples so it really isn’t practical to use.

It is possible that someday this or similar kinds of tests of brain receptor function might help distinguish patients with MDD from those with schizophrenia. Such tests might also help doctors identify which receptors are functioning abnormally in which patients. Knowing the receptor abnormalities might help doctors choose what medicine to use.

However, similar biological tests have been studied before and similarly been found promising. But so far, none have been reliable enough and easy enough to be put into regular use.

What’s the bottom line?

This research provides useful scientific information on brain chemistry in people with psychiatric disorders, but the test itself is not going to help doctors and patients today.