Researchers from Emory University School of Medicine in Atlanta have discovered that a blood test may help determine which patients with depression will respond best to treatments with ketamine, an anesthetic which has in recent years proved effective at helping people with treatment-resistant depression.

"This could be an important step toward personalizing treatment for depression,” Dr. Ebrahim Haroon, a psychiatry and behavioral sciences professor at Emory University School of Medicine, told Al Jazeera. “Traditionally, with regards to the treatment of depression, it’s just a guessing game. You give them [a patient] a medication, and it doesn’t work, and you try another.”

“Identifying the right biochemical or blood markers that will tell us that this medication is good for this patient is a good thing,” Haroon said.

Ketamine, a drug sometimes used in veterinary medicine, gained notoriety as a hallucinogenic party drug in the 1990s rave scene (PDF). But scientists have been surprised to discover that the drug has helped provide immediate relief for some patients who suffer from depression and did not respond to multiple antidepressants or talk therapy.

Ketamine works to interfere with a neurotransmitter called glutamate, which plays a role in mediating the brain’s synaptic impulses. Some scientists believe that by blocking glutamate receptors, ketamine helps build more connections between nerve cells in the brain, reversing some effects of depression.

Depressed people have shown increased levels of glutamate in certain parts of their brains, a situation that can work to damage the neurons over time, explained Haroon, the lead author of the study, which was published Tuesday in the journal Molecular Psychiatry.

A subset of people suffering from depression — about 20 to 30 percent, Haroon said — also have elevated levels of inflammation in their bodies, meaning their immune systems are kicked into high gear to respond to a perceived attack on the body. For example, previous research has shown increased levels of inflammation markers in the blood and cerebrospinal fluid of some patients suffering from depression. It is these patients with heightened inflammation, Haroon said, who typically do not respond to the usual depression treatments such as antidepressant medications.

But not much is known about the relationship between inflammation, higher glutamate levels and depression.

The Emory scientists wanted to measure that relationship, so they examined 50 patients who were diagnosed with depression but not taking medication for it. The scientists determined patients’ levels of inflammation by testing their blood for presence of a substance called C-reactive protein (CRP).

They also tested glutamate levels in the basal ganglia, a portion of the brain related to motor control and motivation.

They found that the presence of the CRP in the blood, which indicates inflammation, as well as elevated levels of glutamate in the basal ganglia were both linked to patients’ reporting of anhedonia — an inability to experience pleasure, a key symptom of depression — and slow motor function, which is measured by a finger-tapping test and is also linked to depression.

This relationship, Haroon said, could indicate that inflammation in the body may work to drive up glutamate levels, which in turn causes depression, though he says more research needs to be done.

More importantly, the blood tests showing inflammation markers could be a good way to determine candidates for glutamate-related depression treatments such as ketamine.

About 7 percent of American adults, according to the World Health Organization, have suffered within the last year from a major depressive episode, which is marked by a depressed mood, difficulties concentrating and changes in eating and sleeping habits lasting for a period of two weeks or longer.