Well their sodium levels can be affected if they are ill.  I guess they check those alongside the other tests because if renin is out, they would have a better picture as the electrolytes can be affected with the children that just have CAH I would have thought.  Hydrocortisone does have some mineralcorticoid properties.  If she grows out of a dose, it would possibly show in renin and 17 OHP and androstenedione.  All these being elevated would indicate that she has either had a period of sickness, grown out of the dose, or been non-compliant on a few.  If they only checked one, and not the others they would not get the same confirmation.  Taking three to my mind is really a good idea.  You are always going to have atleast two others to go by to confirm what is happening if one test is affected?

Renin is checked for Ash every 6 mths.  He also has 17 OHP, androstenedione and creatinine.