Blood test for phenylketonemia

Reference values (norm) for the concentration of phenylketones in the blood of children are up to 121 μmol/l (up to 2 mg%).

Phenylalanine metabolism disorder is a very common congenital metabolic disorder. Due to a defect in the phenylalanine hydroxylase gene (PHA gene), enzyme deficiency develops, and as a consequence, a block occurs in the normal conversion of phenylalanine into the amino acid tyrosine. The amount of phenylalanine in the body accumulates, and its concentration in the blood increases by 10-100 times. Then it turns into phenylpyruvic acid, which has a toxic effect on the nervous system. In this regard, early diagnosis of phenylketonuria is of great importance, since the long-term existence of phenylketonemia leads to a violation of the child's mental development. The accumulation of phenylalanine in the body occurs gradually and the clinical picture develops slowly.

Blood tests are performed within the next 48 hours (2-5 days) after the newborn has received milk (a source of phenylalanine). A disk of filter paper soaked in a few drops of capillary blood and control disks containing different amounts of phenylalanine are placed on a plate with a nutrient medium seeded with bacteria of the phenylalanine-dependent strain of Bacillus subtilis. The bacterial growth zone around the disk soaked in blood is proportional to the concentration of phenylalanine in the newborn's blood.

In addition to phenylalanine hydroxylase defect, transient tyrosinemia of the newborn, which probably results from inadequate tyrosine metabolism, can lead to hyperphenylalaninemia.

Types of hyperphenylalaninemia

Type	Blood phenylalanine concentration, mg%	Defective enzyme	Treatment
Classical phenylketonuria	>20	Phenylalanine hydroxylase	Diet
Atypical phenylketonuria	12-20	Phenylalanine hydroxylase	Diet
Persistent mild hyperphenylalaninemia	2-12	Phenylalanine hydroxylase	Diet
Transient tyrosinemia	2-12	β-hydroxyphenylpyruvate dioxygenase deficiency (etc.) Secondary due to vitamin C deficiency	Vitamin C, Low Protein Blends
Dihydropteridine reductase deficiency	12-20	Dihydropteridine reductase	Dopa, hydroxytryptophan
Defects in biopterin synthesis	12-20	Dihydropteride synthetase	Dopa, hydroxytryptophan
Transient hyperphenylalaninemia	2-20	Unknown	No

The basis of treatment for patients with phenylalanine hydroxylase deficiency is dietary phenylalanine restriction. 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%.

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