How to diagnose phenylketonuria?

Written by Yan Xin Liang

Pediatrics

Updated on April 03, 2025

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Phenylketonuria is a treatable hereditary metabolic disease, where our primary goal is early diagnosis and treatment. Currently, a universal newborn screening system has been widely implemented. The standard procedure requires that after three days of breastfeeding, peripheral blood is collected from the newborn's heel, dropped onto specialized blood collection filter paper, and, once dried, sent to a screening laboratory to measure the phenylalanine concentration. If the phenylalanine concentration exceeds the cutoff value, further differential diagnosis and confirmation are conducted. Treatment typically starts two to three weeks after birth, and the prognosis is good. Additionally, diagnostic tests for phenylketonuria include urine ferric chloride, 2,4-dinitrophenylhydrazine tests, urinary purine profile analysis, and tetrahydrobiopterin loading test. Another method is through DNA analysis.

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Written by Yan Xin Liang

Pediatrics

48sec

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Phenylketonuria lacks what enzyme?

Phenylketonuria is an autosomal recessive genetic disorder primarily caused by mutations in the phenylalanine hydroxylase gene. This leads to reduced activity of phenylalanine hydroxylase, accumulation of phenylalanine and its metabolites in the body, and the onset of the disease. Phenylketonuria is the most common congenital amino acid metabolism disorder. Clinically, it is mainly characterized by intellectual developmental delays, light skin and hair pigmentation, and a mousey urine odor. The incidence of this disease varies by ethnicity and region. In China, the incidence is approximately 1 in 11,000.

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Written by Zeng Hai Jiang

Pediatrics

43sec

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What foods should children with phenylketonuria eat?

Phenylketonuria is a common amino acid metabolic disorder, an autosomal recessive genetic disease, and a type of hereditary metabolic disease that can be treated through dietary control. All natural foods contain certain amounts of phenylalanine. Therefore, once diagnosed, children should stop consuming natural diets and start treatment with a phenylalanine-restricted diet, including low-phenylalanine yogurt. Treatment should continue at least until the age of 12. When the concentration of blood phenylalanine is controlled at an ideal level, gradually add small amounts of natural food, adhering to the principle of low protein and low phenylalanine in the added foods.

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Written by Zeng Hai Jiang

Pediatrics

38sec

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Phenylketonuria treatment duration before discontinuation

Phenylketonuria is the first genetic metabolic disease that can be treated through dietary control. Once diagnosed with phenylketonuria, active treatment must be given, ceasing a natural diet and implementing a low-phenylalanine diet. Generally, a low-phenylalanine milk formula should be administered at least until the age of three. When the concentration of phenylalanine in the blood is adjusted to an ideal range, a small amount of natural diet can gradually be reintroduced. The reintroduced diet should still adhere to low-protein and low-phenylalanine standards.

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Written by Yan Xin Liang

Pediatrics

1min 2sec

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Do children with mild phenylketonuria need treatment?

Phenylketonuria is a common amino acid metabolic disease, primarily due to the deficiency of phenylalanine hydroxylase, preventing phenylalanine from being converted into tyrosine. This leads to the accumulation of phenylalanine and its keto acids in the body, which are then excreted in large amounts in the urine. It is a relatively common genetic amino acid metabolic disorder and is inherited as an autosomal recessive trait. Even in cases of mild symptoms, treatment is still required. The main treatment is a low-phenylalanine diet, which involves dietary control, as phenylalanine is an essential amino acid for protein synthesis. Special phenylalanine-formula milk powder is fed to infants, and at this stage, it is appropriate to add some complementary foods such as starchy vegetables and fruits, focusing on low-protein foods. The goal of dietary treatment is to maintain an appropriate concentration of phenylalanine in the blood.

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Written by Yan Xin Liang

Pediatrics

41sec

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Why does phenylketonuria lead to intellectual disability?

Phenylketonuria is a common amino acid metabolic disease, mainly due to the deficiency of phenylalanine hydroxylase, which prevents phenylalanine from being transformed into tyrosine, leading to the accumulation of phenylalanine and its keto acids. Due to the lack of tyrosine in the body, it affects the growth and development of the brain, resulting in intellectual developmental delays. Therefore, children with phenylketonuria often exhibit intellectual disabilities, as well as some psychiatric and neurological symptoms, such as increased muscle tone, restlessness, hyperactivity, abnormal behavior, and heightened excitability.