The disease is a devastating one, and, if left untreated, babies of only a few weeks old begin to suffer severe neurological damage. All newborn babies in the United States and in most other developed countries are therefore tested for phenylketonuria within the first week of life. This represents the largest genetic screening program carried out by the medical profession. The test, often called the heel-prick test or more correctly the Guthrie test after the scientist who developed it, is relatively simple. A drop of blood taken from a baby's heel is dried onto a small filter paper disk. Disks from hundreds of infants can be tested at the same time by placing their disks onto an agar plate containing bacteria that require phenylalanine for growth. If the bacteria grow, then the baby is at risk from phenylketonuria and blood from the baby will be retested a few days later to ensure the infant does indeed suffer from phenylketonuria. The Guthrie test is simple and cost-effective and can test for phenylketonuria irrespective of the mutation in DNA that has caused the problem.

If detected in the first weeks of life the prognosis for the patient is good. Affected infants are fed a strict diet that provides just enough phenylalanine for protein synthesis but no more. This treatment, carried on to maturity, is very effective and individuals develop normally.

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