Blood test for phenylketonemia

Reference concentrations of phenylketones in blood in children - up to 121 μmol / l( up to 2 mg%).

Disturbance of the metabolism of phenylalanine refers to a very common congenital metabolic disorder. Due to the defect of the phenylalanine hydroxylase gene( phage gene), the enzyme deficiency develops, and as a result, the block enters the normal conversion of phenylalanine to the amino acid tyrosine. The amount of phenylalanine in the body accumulates, and its concentration in the blood increases by 10-100 times. Further, it turns into phenylpyruvic acid, which has a toxic effect on the nervous system. In connection with this, early diagnosis of this disease is of great importance, since the prolonged existence of phenylketonemia leads to a disruption in the mental development of the child. The accumulation of phenylalanine in the body occurs gradually and the clinical picture develops slowly.

The blood test is performed within the next 48 hours( 2-5 days), after the newborn has received milk( a source of phenylalanine).A disk of filter paper moistened with a few drops of capillary blood and a control disc containing various amounts of phenylalanine is placed on a plate of culture medium sown with bacteria of the phenylalanine-dependent Bacillus subtilis strain. The area of ​​bacterial growth around the disc moistened with blood is proportional to the concentration of phenylalanine in the blood of the newborn.

In addition to the defect of phenylalanine hydroxylase to hyperphenylalanineemia, transient tyrosinemia of the newborn can result, which is likely to result from inadequate tyrosine metabolism. The main types of hyperphenylalanineemia are presented in the table.

Table Types of hyperphenylalaninemia

Table Types of hyperphenylalanineemia

End of Table.

End of Table.

The basis for treating patients with phenylalanine deficiency of hydroxy-lysine is the restriction of phenylalanine in the diet. With an adequately selected diet, the concentration of phenylalanine in the blood should not exceed the upper normal level or be slightly below normal.

In patients with transient tyrosinemia, the diet is selected so that the concentration of tyrosine in the blood is between 0.5 and 1 mg% [Prieto J. et al., 1992].