Blood test for phenylketonemia
Last reviewed: 23.04.2024
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The reference values (norm) of concentration of phenylketones in the 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 diagnostics of phenylketonuria 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 carried out within the next 48 hours (2-5 days), after the newborn has received milk (a source of phenylalanine). On a plate with a culture medium seeded with bacteria of the phenylalanine-dependent strain of Bacillus subtilis, a disc is placed from filter paper moistened with several drops of capillary blood and control discs containing different amounts of phenylalanine. 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 newborns may result, which is likely to result from inadequate tyrosine metabolism.
Types of hyperphenylalaninemia
|
A type |
The concentration of phenylalanine in the blood, mg% |
Defective enzyme |
Treatment |
|
Classical phenylketonuria |
> 20 |
Phenylalanine hydroxylase |
Diet |
|
Atypical phenylketonuria |
12-20 |
Phenylalanine hydroxylase |
Diet |
|
Persistent pulmonary hyperphenyl-alaninemia |
2-12 |
Phenylalanine hydroxylase |
Diet |
|
Transient tyrosinemia |
2-12 |
Insufficient β-hydroxyphenylpyruvate dioxygenase (and others) Secondary as a result of a lack of vitamin C |
Vitamin C, a mixture of low protein |
|
Insufficient dihydropteridine reductase |
12-20 |
Dihydropteridine reductase |
Dopa, hydroxytryptophan |
|
Defects of biopsy synthesis |
12-20 |
Dihydropeteride-insintetase |
Dopa, hydroxytryptophan |
|
Transient hyperphenyl-alaninemia |
2-20 |
Unknown |
No |
The basis for treating patients with phenylalanine hydroxylase deficiency 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%.
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