The gold standard for diagnosing depression is a clinical interview with a mental health professional – a face to face discussion where they will review with you all the symptoms that you’ve been having. They’ll review your prior history and try to put all the pieces together to come up with a diagnosis. Sometimes they may provide screening questionnaires where you check off boxes to calculate a total score, but that really should never be the substitute for an actual face to face discussion to really elucidate what their diagnosis is. There is no lab test that can definitively diagnose depression. Nor is there any imaging study that can definitively diagnose depression. That being said, doctors will often order a full lab panel of blood tests at a first visit to rule out any contributory causes. So for example, if your depression is being caused by too little thyroid hormone, it would be important to know and to check that to rule that out. Therefore at the first visit, your doctor will probably want to see what’s called a TSH – a check of the basic thyroid functioning in the blood.