A Blood Test to Diagnose Depression in Teens?

This article is about how blood tests can show depression in teens.  According to the the site, it says that diagnosing depression in teens can be difficult because it's hard to distinguish between mental illness and mood swings.  Currently, a depression diagnosis in teens relies on their descriptions of symptoms and their physician's subjective observations.  The breakthrough with the blood test can lead to earlier diagnoses that may not only allow for better treatment but also help to lessen the stigma associated with mental illness.

This article intrigues me because I've always wondered how doctors diagnose someone with depression.  I'm kind of confused at the fact that they would take blood in order to decide.  I wonder if there's some kind of substance in the blood that only appears when someone is in a depression.

Sifferlin, A. (2012, April 17). A blood test to determine depression in teens?. Retrieved from http://healthland.time.com/2012/04/17/a-blood-test-to-diagnose-depression-in-teens/?iid=hl-main-