90% of patients with CAH have mutations in their 21-beta-hydroxylase gene. This gene produces an enzyme which converts 17-hydroxy progestrone into 11-deoxycortisol which is then converted to cortisol by the enzyme 11-beta hydroxylase. Therefore when the blood tests measure the 17-hydroxyprogesterone levels, they are looking at the product that ends up accumulating because of the block made by the enzyme deficiency. This hormone is produced very fast in response to stress, ( in non-CAH patients allowing for fast production of cortisol to respond to "stress,") and thus by itself is not a totally reliable test of how the overall dosage of the hydrocortisone should be. Another hormone which should be measured is called androstenedione or delta 4 steroids, this hormone is what the 17-hydroxyprogesterone is converted to, and is produced very slowly, and along with the 17-hydroxy test is the more accurate way of testing the dosage. The other commone form of CAH which comprises 10% of CAH patients have the 11-beta hydroxylase deficiency. This enzyme is one step further down the pathway. Although the clinical manifestation of both forms and the treatment for both forms are the same, the tests may vary slightly. In the steroid pathway there are other enzyme deficiencies that may manifest themselves as CAH, but those are by far very rare.