Just to be clear, although most people don't know this, most medical diagnoses don't have a specific lab test to confirm a medical diagnosis. It's a common misconception that medicine has tests for every disease. They don't.

What they do have is a constellation of symptoms, some of which can be measured through a lab test, and if you put all those together, it leads a physician to a handful of possible diagnoses. The whole TV show House MD was based around the more rare of these kinds of diagnoses.

So I would just say don't worry about the specific mechanics of diagnosing a mental disorder. What we do nowadays is really good enough in most cases to help people find an effective treatment that works for them.