Four Consecutive False Negative Newborn Screens in a Patient with Classical Congenital Adrenal Hyperplasia: A Case Report

21-hydroxylase deficiency is the most common cause of congenital adrenal hyperplasia (CAH). Salt-wasting CAH can present with life-threatening salt-wasting crises, underscoring the importance of universal newborn screening. We present a patient diagnosed with classical CAH despite four negative newborn screen results (NBS) results. A male infant was born at 35 weeks gestation with birthweight 1470 grams following signs of placental insufficiency. While hospitalized in the neonatal intensive care unit (NICU), four NBS samples from days of life 2 to 38 were all within normal range, including on repeat analysis via fully integrated fluoroimmunoassay. After initially normal biochemical testing, hyponatremia and hyperkalemia developed by day of life (DOL) 26, responsive to sodium chloride supplementation. Following recurrent hyponatremia after trial off supplementation after day of life 50, 17-hydroxyprogesterone as measured via liquid chromatography-tandem mass spectrometry were reported at two different labs as 10,900 ng/dL and 11,200 ng/dL (<2 ng/dL at 50 DOL). Subsequent testing identified deletion of one <i>CYP21A2</i> allele and a <i>I172N</i> mutation in the second. This report illustrates the importance of maintaining a high index of suspicion for classic forms of CAH in infants with persistent electrolyte disturbances despite negative NBS results.