The immediate precursor in classic CAH and the substrate for CYP21 is 17- hydroxyprogesterone (17-OHP). The measurement of 17-OHP in the newborn blood spot can discriminate infants with salt-wasting or Simple Virilizing CAH from non-affected infants. The newborn screening test usually does not detect attenuated or late onset nonclassical CAH patients. When values exceed the normal range, 17-OHP analysis is repeated using organic extraction to remove interfering substances. The normal values for 17-OHP vary with birth weight and gestational age, and cutoffs should be adjusted accordingly.
Serum confirmation tests include a repeat 17-OHP value, other precursors to be certain that a mildly elevated 17-OHP is not caused by another form of CAH (e.g., 11-hydroxylase deficiency), and tests related to salt loss, such as serum Na and K, and renin activity. Confirmatory DNA testing is also available.