Test code links throughout this Guide refer to the corresponding test in the User's Guide. A dihydropteridine reductase deficiency or defect in biopterin synthesis will also produce phenylalaninemia. Elevated phenylalanine can be seen in a variety of conditions, including transient hyperphenylalanemia of the newborn, transient tyroinemia of the newborn, prematurity, tyrosinemia sepsis, severe burns, and liver disease. The buildup of phenylalanine in tissue causes progressive mental retardation, starting in the second or third week of life. If infants with this disorder are immediately placed on low phenylalanine diets, damage is minimal.
Untreated patients become severely retarded.
Since phenylalanine is also necessary for normal growth and development, it is critical to monitor levels in the PKU patient. Phenylalanine levels are also checked to confirm positive PKU screens on newborns. It is most critical to monitor women with PKU who bear children, as the fetus may be affected by either very low or high levels of phenylalanine. Many amino acids require a plasma specimen for accurate analysis, but either plasma or serum is acceptable for phenylalanine diet monitoring. To facilitate small specimen volumes, the analysis does not use an internal standard, and the coefficient of variation is therefore larger than in standard plasma amino acid testing.