To obtain adequate sensitivity, the cutoff levels for 17-OHP are typically set low enough that approximately 1% of all tests are reported as positive. Despite the high accuracy of the screening test and given the low prevalence of CAH (about one in 10,000 births), only approximately one in every 100 neonates with a positive screening test will have CAH. Much of the expense of following up positive newborn screening tests could be avoided with a second level of more specific screening. Both biochemical and molecular genetic approaches have been proposed.
The mineralocorticoid pathway starts with 21-hydroxylation of progesterone to form deoxycorticosterone (DOC). The enzyme in this reaction, 21-hydroxylase, is encoded by the CYP21 gene. 11 , 12 Deoxycorticosterone is then converted to corticosterone through the action of 11β-hydroxylase. There are two distinct 11β-hydroxylase isoenzymes, both of which are encoded by two genes, CYP11B1 and CYP11B2 . 13 Corticosterone is hydroxylated at carbon 18 to form 18-hydroxycorticosterone, which is transformed to aldosterone by removal of two hydrogens (oxidation) at carbon 18. These two reactions are catalyzed by 18-hydroxylase and 18-hydroxysteroid dehydrogenase, respectively, which are encoded by the same gene, CYP11B2 . Transcription of CYP11B1 is regulated primarily by ACTH, whereas angiotensin II regulates CYP11B2 transcription. 14 , 15 Similarly, the glucocorticoid pathway begins with 17α-hydroxyprogesterone, which is converted to deoxycortisol and subsequently to cortisol by 21-hydroxylase and 11β-hydroxylase, respectively, in the same manner as the conversion of progesterone to corticosterone. A deficiency of 21-hydroxylase, 11β-hydroxylase, or 3β-HSD in the adrenals may result in congenital adrenal hyperplasia and female pseudohermaphroditism, manifested as a masculinized female fetus.
As a mitochondrial P450 system, P450c11 is dependent on two electron transfer proteins, adrenodoxin reductase and adrenodoxin that transfer 2 electrons from NADPH to the P450 for each monooxygenase reaction catalyzed by the enzyme. In most respects this process of electron transfer appears similar to that of P450scc system that catalyzes cholesterol side chain cleavage.  Similar to P450scc the process of electrons transfer is leaky leading to superoxide production. The rate of electron leakage during metabolism depends on the functional groups of the steroid substrate.