Within a few days of birth, all babies in the United States are tested for phenylketonuria (PKU). The test for phenylketonuria uses two or three drops of blood taken from your baby's heel to measure phenylalanine levels. Phenylalanine blood levels are higher in babies with PKU.

Most doctors will take the blood sample after your baby has been fed, 24 to 48 hours after birth.² It can take a few days for phenylalanine to build up in a newborn's body, so testing before 24 hours can give untrue results (false negatives).

Periodic blood tests to measure phenylalanine levels are recommended for babies and pregnant women with PKU. Babies may get tested as often as once a week for the first year and then once or twice a month throughout childhood. Pregnant women with PKU should be tested at least once a week.

PKU is an autosomal recessive disease, which means it will only develop if a person inherits the gene from both parents. If the PKU gene is inherited from only one parent, the person will be a carrier of the gene for PKU but will not develop the disease. The risk of passing on an autosomal recessive disease depends on your family medical history. Ask your doctor about a genetic test if you are interested in confirming whether you carry the gene for PKU.