Congenital adrenal hyperplasia (CAH), a disorder caused by a deficiency of the 21-hydroxylase enzyme, is the most common inborn error of the adrenal steroid pathways. Early diagnosis of CAH can be lifesaving, and screening for CAH in newborns by measuring 17-hydroxyprogesterone (17OHP) or other steroids has become a routine part of many programs (1)(2). These steroid hormones have been measured by fluorometry (3)(4), immunoassay (5)(6)(7)(8), and HPLC (4)(9)(10). Most methods are affected by interferences or cross-reactivity with other steroids. Currently, neonatal screening and monitoring for CAH use immunoassays (3)(4). This approach, although practical, lacks specificity because cross-reacting congeners are inseparable from 17OHP in the direct assay.