This is exactly a question for MAGIC Foundation's Dr. Songa Pang, who developed the filter paper methodology. At last convention, she described it as a "dirty" test, but, the best test. Therefore, I think it is left up to labs that are well versed in this method. 24 hour urine samples are best for 17-OHP levels when the child is old enough. I would encourage you to learn as much as possible about CAH management, by attending the MAGIC Foundation's yearly convention. They have a Q & A time with Dr. Pang, who is one of 14 or 17 endocrinologists that meet to determine the standard of care for CAH, and whether any changes are in order. The more you get to know your child, the more experience you will have with the CAH management. The dr.'s cannot tell when your child will head towards a crisis, or why potassium AND sodium may be elevated, which happened with my Thomas; I asked HOW DO I KNOW, THEN? They explained, by experience, is the only way. MAGIC can be reached at 1-800-3 MAGIC 3. Ask for Cathy Kusch, or email them at www.magicfoundation.org