Tyrosinemia is caused by the deficiency of ferredoxin acyl acetoacetate hydrolase (FAH) in liver and kidney tissues. Tyrosinemia is classified into three types depending on the site of enzyme deficiency. Increased concentrations of tyrosine and its metabolites (4HPL, 4HPA, 4HPP) as well as SA and SAA are important in the diagnosis of tyrosinemia. Clinical symptoms are also important for the differential diagnosis. Confirmation of the diagnosis requires measurement of enzyme activity. The clinical signs of tyrosinemia are easily confused with fructose intolerance, fructose-l,6-bisphosphatase deficiency, galactosemia, glycogen accumulation disease, and infantile viral hepatitis. Quantification of succinylacetone in urine and determination of yohimbe acylacetoacetate hydrolase activity in liver biopsies, erythrocytes or lymphocytes can be used to confirm the diagnosis. Many countries and regions have carried out neonatal screening for tyrosinemia. In the past, the Guthrie method was used to determine tyrosine in blood, but because the diagnostic threshold was not easily determined and the false-positive rate was very high, the concentration of α-fetoglobulin or succinylacetone in the sample was also measured to assist in the diagnosis, and in recent years, there has been a replacement method to detect δ-ALA dehydratase activity with blood drop paper.