The ACTH stimulation test is used to diagnose late onset CAH. I don’t know if it is used for classic CAH (apparently not). From what I’ve read, ACTH is administered through iv and your 17 OHP is checked to see if it spikes up (normally, it shouldn’t). I guess if your 17 OHP is high enough to start with, as in classic CAH, there is no need for the stimulation test.