Student A: I can’t wait until the psych test is over. I’m so sick of memorizing diagnoses – I mean, I’m sorry, but the DSM is just so arbitrary it’s stupid.
Me: Yeah, but until we get a blood test for depression, “general consensus of professionals” is the best we can do, you know?
Student B: Hey guys. How’s studying going?
Student A: The DSM is insane.
Student B: … according to what, exactly?
Action Potential 
You can’t say the DSM is insane until you can conclude that there is not some other underlying organic process/substance abuse or it’s behavior is not better explained by another condition.
There are plenty of other “medical” diseases where criteria for diagnosis has been decided by consensus. Most of the rheum ones, for instance. Sure, some of the tests are blood tests, but that doesn’t make it more “real”. All tests are associated with a sensitivity and specificity, which means that diagnosis of lots of conditions is not an exact science.
Yeah, I agree with you guys and am very much pro-DSM. That last sentence was just pretty damned funny to me.