The neonatal screening test for homocystinuria primarily measures methionine by using a dried blood specimen. We investigated the incidence and clinical manifestations of homocystinuria, isolated hypermethioninemia, and transient hypermethioninemia among patients with hypermethioninemia on a neonatal screening test.

We performed a retrospective study of 58 patients transferred to Shoonchunhyang Hospital because of hypermethioninemia on a neonatal screening test between January 1996 and August 2009. We analyzed the level of amino acid from plasma and urine, as well as blood homocysteine.

Almost half of the 58 patients were identified as normal. Whereas only 3 (5.1%) patients were identified as having homocystinuria, about 20.7% (12 cases) of the patients had isolated hypermethioninemia. The ages of these two groups at initial detection of hypermethioninemia on plasma amino acid analysis were 50.0±22.5 days and 34.9±13.5 days, respectively. Both groups were put on diets, and they showed a normal developmental course as a result of early diagnosis and treatment.

Hypermethioninemia without homocystinuria, referred to as isolated hypermethioninemia, was also detected. Thus, the impact of hypermethioninemia on a neonatal screening test should be carefully evaluated through analysis of amino acid levels from blood and urine, and we need to detect and treat an early stage of isolated hypermethioninemia as well as homocystinuria.