How is phenylketonuria tested?

Written by Yan Xin Liang

Pediatrics

Updated on January 23, 2025

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Phenylketonuria is a treatable inherited metabolic disease, and early diagnosis is essential for early treatment. Currently, we have generally implemented a newborn disease screening system that requires collecting peripheral blood by pricking the heel of newborns after three days of breastfeeding, dropping the blood onto specialized filter paper and sending it to a screening laboratory for phenylalanine concentration measurement. If the concentration exceeds the confirmed value, further differential diagnosis and confirmation are needed. If treatment can begin early, especially within two to three weeks after birth, the prognosis is generally good. Normally, the concentration is less than 120 µmol/L; a fetal concentration of 1200 µmol/L would be considered mild phenylketonuria.

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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

55sec

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Phenylketonuria Test Methods

The examination methods for phenylketonuria primarily include newborn screenings usually performed three to seven days after birth by drawing a blood sample from the heel and placing drops on specialized blood collection filter paper. Once dried, the sample is sent to a screening laboratory to determine the concentration of phenylalanine. If the phenylalanine concentration exceeds the cutoff value, further examinations and confirmatory tests are necessary, with the confirmatory test involving the measurement of phenylalanine concentration. Another method involves the analysis of urinary pterin profile, mainly used to differentiate dihydrobiopterin reductase deficiency. Additionally, gene analysis can be applied to detect mutations in genes such as phenylalanine hydroxylase, aiding in genetic diagnoses and prenatal diagnoses.

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Written by Hu Qi Feng

Pediatrics

45sec

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Phenylketonuria Clinical Symptoms

Phenylketonuria may appear normal at birth, but symptoms usually start to appear between three to six months, with the symptoms becoming most evident at one year old. These are often manifestations of the nervous system, with prominent developmental delays in intelligence. The IQ is often lower than normal, and there may be abnormal behaviors, minor epileptic seizures, and, in a few children, increased muscle tone and hyperactive tendon reflexes. Due to insufficient melanin synthesis a few months after birth, affected children’s hair color changes from black to yellow, their skin is fair, and skin eczema is relatively common. Due to the elevated excretion of phenylacetic acid in urine and sweat, there is a distinct mouse-like urine smell.

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Written by Yan Xin Liang

Pediatrics

1min 3sec

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Phenylketonuria positive means what?

Phenylketonuria is an autosomal recessive inherited disease and is the most common congenital amino acid metabolism disorder. Generally, if the activity of phenylalanine hydroxylase in the body is reduced, or if there is a deficiency in its coenzyme tetrahydrobiopterin, the concentration of phenylalanine in the blood and tissues will increase, and phenylpyruvic acid, phenylacetic acid, and phenyllactic acid in the urine will also increase. Generally, newborns may not show any special clinical features at first. Often, the first test results may be high due to maternal influence or dietary factors, which can cause these elevated indicators in newborns. If the second test is normal, there usually isn’t a significant problem. A third test can also be performed; if the third test still shows no issues, then phenylketonuria can be ruled out. Generally, if phenylketonuria is diagnosed, it should be treated promptly and properly.

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Written by Wu Ben Rong

Pediatrics

52sec

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Is phenylketonuria related to a baby's face turning dark?

Baby's dark complexion is not directly related to phenylketonuria (PKU). Phenylketonuria is a relatively common amino acid metabolism disorder caused by a deficiency in the enzyme needed for the metabolic pathway of phenylalanine, preventing it from being converted into tyrosine. This leads to the accumulation of phenylalanine and keto acids in the body, which are then excreted in large amounts through urine. Children with phenylketonuria may have lighter hair and skin color within months of birth due to insufficient melanin synthesis. Typically, their skin is very pale; therefore, if a child has darker facial skin, it is not related to phenylketonuria. This condition might be due to genetic factors or prolonged exposure to the sun among other reasons.