False negatives at neonatal screening for congenital adrenal hyperplasia in two siblings with 21-hydroxylase deficiency.

We report on two siblings with classic simple virilizing 21-hydroxylase deficiency whose neonatal screening for serum 17alpha-hydroxyprogesterone (17-OHP) gave normal results. The proband, a girl with clitoromegaly whose screening 17-OHP value had been 9.2 ng/ml, was diagnosed at the age of 6 months, and her elder brother with the initial screening level of 15.7 ng/ml was diagnosed at the age of 6 years due to precocious puberty. Although the occurrence of false-negative cases is extremely rare, it can happen in a simple virilizing form of 21-hydroxylase deficiency. This experience informs that normal results for neonatal screening cannot be an excuse for not evaluating siblings of the proband with congenital adrenal hyperplasia.