Patients with PKU caused by both classic and cofactor deficiencies have hyperphenylalaninemia, but those with hyperphenylalaninemia do not necessarily cause PKU, so PKU should be differentiated from other hyperphenylalaninemic patients. Although the cause of transient hyperphenylalaninemia is also due to a defective PAH, it is not due to a mutation in the PAH gene, but to immature PAH, resulting in an elevated blood phenylalanine concentration of 1.22 mmol/L. However, over time, the blood phenylalanine concentration can decrease to normal, and can be differentiated by following up the blood phenylalanine level. Aminotransferase hyperphenylalaninemia is caused by a deficiency of phenylalanine aminotransferase. This condition does not cause phenylketonuria, and the blood phenylalanine level is normally normal, but the blood phenylalanine concentration increases when eating a high protein diet, and the level of phenylalanine metabolites is also normal. Mild PKU can also be differentiated from cofactor-induced PKU only by genetic diagnosis and determination of blood tyrosine levels or phenylalanine loading test to determine the ratio of phenylalanine to tyrosine.