The indication of weakly positive phenylketonuria suggests a re-examination of blood phenylalanine levels. Generally, there are no obvious specialized clinical symptoms of phenylketonuria in the newborn period. Some newborns may experience feeding difficulties, vomiting, or irritability, and the manifestations of phenylketonuria gradually appear after three months of age. Currently, hospitals routinely screen newborns for phenylketonuria. This screening is conducted 72 hours after birth and must be completed by no later than 20 days post-birth. If the result of the blood phenylalanine test is weakly positive or positive, further testing is generally recommended to determine whether the infant has phenylketonuria or if other factors are affecting the phenylalanine levels.