There are various congenital enzyme defects of steroid biosynthesis, which cause congenital adrenal hyperplasia (CAH). They are genetically different but are all transmitted in an autosomal recessive mode. The most frequent types are the 21-hydroxylase deficiency (about 80% of all cases) and the 11ß-hydroxylase deficiency (about 15% of all cases). Other types (lipoid adrenal hyperplasia or 20α-hydroxylase deficiency, and 3ß-hydroxysteroid dehydrogenase deficiency) are very rare. The overall frequency of CAH in the general population varies ethnically. 

Individuals with CAH may present with life-threatening salt-wasting crises in the newborn period and incorrect gender assignment of virilized females. Hormone replacement therapy, when initiated early, results in a significant reduction in morbidity.                                                                                   

 17α-OH-progesterone, a precursor of cortisol, is increased in both 21- and 11ß-hydroxylase deficiency, but not in the other types. Its determination is thus useful as a screening method for the two most frequent types of CAH or about 95% of all cases. 17α-OH-progesterone is also produced in the placenta as a metabolite of progesterone. The levels are therefore high in umbilical cord blood and in blood drawn from all normal newborns. In normal newborns, however, concentration drops rapidly within the first days of life. In children with CAH, by contrast, the levels stay high or increase even further. At the time, when blood on filter paper is generally collected for screening procedures, the 17α-OH-progesterone levels are still higher than in older infants or children.   

If CAH is detected by the screening procedure, differentiation between 21- and 11ß-hydroxylase deficiency has to be carried out later on. Late onset non-classic CAH is not accurately detected by newborn screening.

Reference: GSP Neonatal 17α-OH- progesterone kit package insert