Most (90%) cases of congenital adrenal hyperplasia (CAH) are due to mutations in the steroid 21-hydroxylase gene (Cyp21). CAH due to 21-hydroxylase deficiency is diagnosed by confirming elevations of 17-hydroxyprogesterone (OHPG) and androstenedione (ANST) with decreased cortisol. By contrast, in two less common forms of CAH, due to 17-hydroxylase or 11-hydroxylase deficiency, OHPG and ANST levels are not significantly elevated and measurement of progesterone (PGSN) and deoxycorticosterone (DOC), respectively are necessary for diagnosis. Since 21-hydroxylase deficiency is more common and results in remarkable increase in OHPG, this test is most commonly ordered compared to other steroid intermediates. Various methods are used in clinical laboratories for the analysis of OHPG in serum or plasma but mass spectrometric methods are considered gold standard method.