Phenylketonuria is a treatable inherited metabolic disease, and early diagnosis is essential for early treatment. Currently, we have generally implemented a newborn disease screening system that requires collecting peripheral blood by pricking the heel of newborns after three days of breastfeeding, dropping the blood onto specialized filter paper and sending it to a screening laboratory for phenylalanine concentration measurement. If the concentration exceeds the confirmed value, further differential diagnosis and confirmation are needed. If treatment can begin early, especially within two to three weeks after birth, the prognosis is generally good. Normally, the concentration is less than 120 µmol/L; a fetal concentration of 1200 µmol/L would be considered mild phenylketonuria.