Newborn screening second-tier tests are confirmatory tests performed when primary screenings, by tandem mass spectrometry or another method, yield a questionable result. The accumulation of Succinylacetone (SUAC) results from the deficiency of the enzyme fumarylacetoacetate hydrolase (FAH) in the tyrosine catabolism pathway. While tyrosine can be determined by routine newborn screening, it is not a specific biomarker for type I tyrosinemia and can often be associated with diffuse and benign transient tyrosinemia of the newborn. The presence of SUAC in blood is pathognomonic for Hereditary Tyrosinemia type I (HT-1).