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CAH Testing

Congenital Adrenal Hyperplasia (CAH) is a group of autosomal recessive disorders caused by defects in the synthesis of cortisol by the adrenal gland. The most common form, accounting for 90% of CAH cases, is the defect in the 21 hydroxylase gene (CYP21) causing absent or decreased production of the enzyme 21 hydroxylase. The pathological effects of the enzyme defects are: excess androgen production causing virilization, decreased production of cortisol leading to excess production of ACTH causing adrenal hyperplasia, and lack of aldosterone production causing salt wasting which can be life threatening. The impairment of 21-hydroxylase results in increased concentration of 17-hydroxyprogesterone (17OHP).

Newborn screening can detect infants with CAH by measuring 17-hydroxyprogesterone in blood spots. However this test has a large number of false positive results due to the lack of antibody specificity and high cross-reactivity. In addition, increased levels of 17-OHP can also be caused by low birth weight or illness. Although correction for birth weight reduces the number of false positives, this test remains the one with the lowest predictive value, when only the concentration of 17-OHP is measured.

In an effort to improve the specificity of newborn screening for CAH, we are using a second tier test for steroid profiling using liquid chromatography-tandem mass spectrometry (LC-MS/MS). Samples with an elevated concentration of 17-OHP are referred for the second tier test. After extraction with organic solvents, the samples are analyzed by LC-MS/MS. The concentration of 17-OHP, androstenedione, and cortisol is measured and the ratio (17-OHP+ androstenedione) /cortisol is calculated. Cut-offs have been established for 17-OHP and the ratio. The use of this second tier test has significantly increased the specificity of the screening and has dramatically reduced the false positive rate.