Hi A.,

PLEASE don’t misconstrue what I said to mean that the ACTH is not a good test...of course it is!  It is probably the BEST test to determine if someone has CAH.  But, I am making my comments specifically to address the situation that you have presented....that of a child who has been treated prenatally with dexamethasone. 

By the time your hypothetical child is born, it has ALREADY been determined that she has CAH.  Therefore, you ALREADY know that she is cortisol deficient.... so, in THIS situation, the information from the ACTH is probably of limited value.  Why run another test to tell you something you already know?  

What you now need to find out is if this child also has salt-wasting tendencies, and to do that, doctors normally look at renin and electrolytes.  But, this is done even in children who are diagnosed via ACTH.  The ACTH confirms cortisol deficiency, the renin and electrolyte numbers confirm aldosterone deficiency. 

To answer your question about the dex affecting ALL the levels:  The dex affects the baby’s adrenal function.  However, renin and electrolytes are not adrenal hormones.  Therefore, they will be unaffected by the dex.