Most of the markers she identified in the adult depression panel are different from those in depressed adolescents. Blood Test to Diagnose Depression in Adults Test identifies nine blood markers tied to depression; predicts who will benefit from therapy. Your doctor may also check your blood glucose levels before prescribing you a medication in blood adults test for depression treat depression. But this blood test could take the guesswork out of diagnosing depression and give further evidence that depression is a physiological illness.