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Inheritance of this disorder results in a build-up of phenylalanine (a protein component) in the blood. It may cause developmental delays, seizures, severe mental retardation, and an unusual mousy odour. Diagnosis is made by routine screening of all babies.

Blood is collected onto a piece of card (Guthrie card), and several spots can be collected at the same time to allow screening for other conditions, such as congenital hypothyroidism. For the diagnosis of PKU, blood phenylalanine concentration is measured.

The condition is controlled by restricting phenylalanine in the diet as soon as possible and certainly, before four weeks of age to avoid permanent brain damage. This dietary restriction must continue throughout the patient's life.

Limitation of PKU test
Collection of an insufficient amount of specimen will affect the test result. The child should be taking a normal diet to prevent an inaccurate result.