Apparently from what we were told when Savannah was having daily monitoring as an infant, it is because it affects the potassium reading.  Their potassium levels are higher in dehydration (someone correct me here if I have it backwards) and the heel prick gives a higher than normal potassium reading even in a non CAH baby. 

In fact, the first blood draw we did when she was released from the hospital after birth, was done by a "new" person, who didn’t know this.  He did the heel prick trying to be nice considering he saw all of the other bruises on the veins on the back of her hands and inside the arm where it bends.  The pediatrician called us on the cell phone immediately after we left and wanted us to get the levels checked again and make sure it wasn’t a heel prick.  That’s when he told us about the potassium reading being off if done through the heel.  So then it meant that she would get pricked twice.  So even though someone was trying to be nice... it’s always best to get it done in the right place. 

As far as the potassium being elevated in when taken in the heel prick... I have no idea why.  That’s a good one for me to look up on a rainy day.  Did I mention I live in a desert?.  Sorry, just giving humor to add a little laughter on a cloudy subject.  Once he gets a little older you might want to consider asking your endocrinologist for a prescription of EMLA cream to numb him for his blood draws.  Lots of mom’s here use it and say it works.  I know you can’t "prep"him with EMLA for an emergency blood draw, but at least it might help you with the scheduled ones.

I know it feels like everyone’s pricking and poking him now but it will lessen up as he gets older.  I hope your son is feeling better soon.  So sorry to hear he is ill.

Blessings,

AdinaH