Not much... ACTH can be a wide reference range and I don’t see how they can use it to gauge what is going on for a child without other blood tests run along side of it.  hey should be paying more attention for example to the androgens and the 17 OHP more than the ACTH.  That can look quite normal even when it is high because ACTH is something that responds to stress and the levels can fluctuate from very low to very high throughout a 24 hour period.  Low in sleep, high first thing in the morning---as does cortisol normally in Non -CAH folks.  So if it was not that high it would for examply give you NO idea of what the androgens were doing because taking a dose of cortisol will make ACTH appear low for hours...yet the androgen levels could still be high due to inadequate long term treatment or control.