Patient's question:
My child underwent newborn disease screening, (heel blood collection) and the results showed a suspicion of phenylketonuria: The first indicator was 2.4 mg/dL (filter paper blood test reference value for PKU), the second recheck indicator was 2.35 mg/dL, and the third indicator was 2.30 mg/dL. Could it be confirmed that my child has phenylketonuria based on these three sets of data?Doctor's answer:
Phenylketonuria (PKU) is one of the earliest identified treatable genetic metabolic disorders. Early diagnosis and prompt treatment can prevent intellectual damage in children and allow them to live a life similar to that of healthy individuals. The diagnosis of PKU primarily relies on the measurement of phenylalanine in the blood, with phenylalanine levels in affected children typically exceeding 20 mg/dL.1. Guthrie Method
This is the earliest and most cost-effective semi-quantitative method for measuring blood phenylalanine. The principle is as follows: Bacillus subtilis (ATCC-6633) requires phenylalanine for growth. On a medium containing β-2-thienylalanine (an inhibitor), Bacillus subtilis cannot grow. When a blood filter paper specimen is placed on the medium, phenylalanine in the blood counteracts the inhibitor, resulting in a clear bacterial growth ring around the paper. Based on the size of the bacterial growth ring, the phenylalanine concentration in the blood filter paper can be determined.
2. Fluorescence Photometric Method
A quantitative method for measuring phenylalanine.
3. Amino Acid Chromatography Method
A simpler quantitative method for phenylalanine using finger or heel blood.
4. Amino Acid Analysis Method
A quantitative method that uses an amino acid analyzer to automatically analyze blood amino acids. It can differentiate amino acid metabolic disorders based on the quantification of phenylalanine, tyrosine, and other amino acids, as well as the ratio of branched-chain to aromatic amino acids. Amino acid analysis is particularly important for distinguishing between different types of PKU and for differentiating hyperphenylalaninemia.
5. Phenylalanine Tolerance Test
Oral administration of 100 mg/kg of phenylalanine, followed by blood phenylalanine measurement 1–4 hours later. If phenylalanine levels are elevated and tyrosine levels are low, the diagnosis is confirmed. Typical PKU children show positive results in the urinary ferric chloride and 2,4-dinitrophenylhydrazine tests, but these tests are easily affected by other factors, have poor stability, a high rate of false positives, and are prone to missed diagnoses. They should only be used as a reference. Classic PKU should be differentiated from various types of hyperphenylalaninemia caused by different gene mutations. Prenatal diagnosis can also be performed.
### Treatment Measures
The low-phenylalanine diet is the only current treatment for classic PKU, with the goal of preventing brain damage. The principle of the diet is to ensure that phenylalanine intake meets the minimum requirements for growth and metabolism. Due to the lack of phenylalanine hydroxylase in the liver, phenylalanine cannot be metabolized normally to tyrosine, leading to its accumulation in the blood and causing neurological damage. Phenylalanine is also metabolized via alternative pathways, producing phenylpyruvate and phenylacetic acid, which are excreted in the urine, giving the child’s urine a mouse-like odor. Phenylalanine is an essential amino acid, and insufficient intake can lead to growth and development delays, potentially resulting in death. Therefore, phenylalanine intake must neither be too high nor too low. Since natural proteins contain 4–6% phenylalanine, the intake of natural proteins must be controlled, and low- or phenylalanine-free (powdered milk) and protein powder should be used as the primary protein sources for PKU children.
80% of total protein intake should come from artificial protein, while 20% comes from natural protein. Sufficient calories must also be provided. During treatment, phenylalanine intake must be strictly limited to prevent abnormal accumulation of phenylalanine and its metabolites, while meeting the body’s needs to ensure normal development. Breast milk remains the best diet for infants, and providing calculated amounts of breast milk is highly beneficial for development, so breastfeeding should never be discontinued. Dietary treatment must also consider individual differences, as the severity of phenylalanine hydroxylase deficiency varies greatly among patients. Therefore, individualized dietary treatment is essential, and meal plans should be tailored and adjusted based on each child’s age, weight, blood phenylalanine concentration, and tolerance (see Tables 1, 2, 3). Generally, infants under one year old should have their meal plans adjusted monthly, children over one year old every two months, and school-aged children every 3–4 months.
When creating meal plans, the daily protein, phenylalanine, and calorie needs are first calculated based on the child’s condition, followed by the arrangement of specific meals. Historically, dietary treatment was thought to be unnecessary after brain development maturity (around age 8). However, recent treatment practices have shown that premature discontinuation can lead to intellectual regression in children and various behavioral abnormalities in adults, especially in female patients who may experience fetal brain damage during pregnancy due to high blood phenylalanine levels. Therefore, international guidelines now recommend at least lifelong treatment, with adjustments allowed after puberty.
In China, low- or phenylalanine-free formulations are available, with the most widely used being special nutritional diets approved by the government for PKU—such as the Weishuo series of low- or phenylalanine-free (powdered milk), protein powder, starch, and beverages. Other manufacturers also produce similar products. The quality of these formulations significantly impacts treatment efficacy, so careful selection is necessary.
To ensure efficacy, blood phenylalanine concentration must be monitored regularly (see Table 4). Normal blood phenylalanine levels range from 1–2 mg/dL. Untreated classic PKU typically has phenylalanine levels >20 mg/dL, most commonly between 20–50 mg/dL, with levels >50 mg/dL being rare. After dietary treatment, blood phenylalanine concentration should be checked 1–2 times per week during the first month and once per month thereafter. The key to dietary treatment is controlling blood phenylalanine levels while ensuring adequate protein and calorie intake. Insufficient protein or calorie intake can lead to malnutrition, causing protein breakdown and elevated blood phenylalanine levels. Regular follow-ups of hemoglobin, albumin, electroencephalography, physical and intellectual development, and blood amino acid analysis (including tyrosine levels and the ratio of branched-chain to aromatic amino acids) may be necessary.
For atypical PKU, in addition to dietary treatment, supplementation with multiple neurotransmitters such as BH4, dopamine, 5-hydroxytryptamine, and folic acid is required. Children with other comorbidities should receive symptomatic treatment. For example, those with epilepsy should begin standardized antiepileptic medication early, and children with eczema may recover after satisfactory blood phenylalanine control. If eczema is severe, topical medications can be used for symptomatic relief. Intellectual delays caused by brain damage are irreversible, but intellectual rehabilitation can lead to varying degrees of improvement, with some even showing significant progress. Families with financial resources may consider intellectual rehabilitation training for their children. For severely intellectually delayed children, the goal is to develop basic self-care skills, while for mild-to-moderately delayed children, life skills training should be provided in addition to developmental support.
Early detection and treatment of PKU yield the best outcomes. Some children can achieve normal intellectual levels, but no treatable child should be abandoned. Past textbooks suggested that treatment was unnecessary for children over 6 months, but experience shows that children of any age can improve intellectually and achieve self-sufficiency after treatment. Suspected cases in outpatient clinics should also undergo screening, diagnosis, and treatment to reduce the population of intellectually disabled individuals.
### Prevention
Avoiding consanguineous marriages, particularly between heterozygotes, is crucial. Newborn screening should be conducted to detect PKU early and initiate treatment to prevent intellectual disability. For families with existing PKU children, prenatal diagnosis can be performed during early or mid-pregnancy by sampling fetal chorionic villi or amniotic fluid for genetic diagnosis. This determines whether the fetus is normal, a carrier, or affected, allowing decisions on continuing or terminating the pregnancy. Currently, approximately 80% of PKU mutations in China are known, while the mechanisms of the remaining 20% remain unclear. Each PKU family has two mutated genes, so genetic diagnosis can yield three results:
1. Both mutated genes are clearly identified.
2. One mutated gene is identified, while the other is unclear.
3. Both mutated genes are unclear.
The first two results allow for prenatal diagnosis. In the third case, under the premise of differentiating atypical PKU, indirect genetic analysis via linkage analysis can be performed.