Citrullinemia is the earliest identifiable biochemical abnormality in neonates with
intrahepatic cholestasis due to a
citrin deficiency (
NICCD) and it has been included in newborn screening panels using tandem mass spectrometry. However, only one neonate was positive among 600,000 infants born in Sapporo city and Hokkaido, Japan between 2006 and 2017. We investigated 12 neonates with
NICCD who were initially considered normal in newborn mass screening (NBS) by tandem mass spectrometry, but were later diagnosed with
NICCD by
DNA tests. Using their initial NBS data, we examined
citrulline concentrations and ratios of
citrulline to total
amino acids. Although their
citrulline values exceeded the mean of the normal neonates and 80% of them surpassed +3 SD (standard deviation), all were below the cutoff of 40 nmol/mL. The ratios of
citrulline to total
amino acids significantly elevated in patients with
NICCD compared to the control. By evaluating two indicators simultaneously, we could select about 80% of patients with missed
NICCD. Introducing an estimated index comprising
citrulline values and
citrulline to total
amino acid ratios could assure
NICCD detection by NBS.