My son has SWCAH but his diagnosis was questioned a couple of years ago so he had the short synacthen (ACTH) test.  He did have to omit his nighttime medication and the test was performed the following morning after an overnight stay in hospital (as a precaution).  The result showed an ACTH level of over 8000, but a zero level of both cortisol and undetectable level of 170HP - this suggested he had adrenal insufficiency but CAH was very unlikely.  The normal result for a SWCAH child would be a very low level of cortisol and a high 17OHP. 

He then went on to have the long ACTH test, where medication was stopped for 4 days and daily injections of ACTH given to stimulate the adrenals - the same results were received.   I never expected him to survive 4 days without medication but apart from a bit of a headache and feeling very tired (due to no cortisol in the system at all) he was fine.  His b/p, electrolytes and blood sugars were checked at regular intervals whilst in hospital, so he was well monitored.   

DNA results shortly afterwards revealed he does have severe SWCAH but for some reason (possibly overtreatment in the past) his adrenal glands had completely switched off, so 17OHP was not being produced either.  I think this is rare and usually a short synacthen is sufficient and results are generally reliable.  Also if done on a hospital admittance the child is not going to come to any harm.