A positive screening result can feel frightening. One thing to know immediately: PKU is highly treatable. Children whose therapy starts early develop completely normally and lead healthy, active lives. In Europe, PKU affects around 1 in 10,000 newborns.
Newborn metabolic screening is a routine procedure — mandatory and free in Austria, Germany, Switzerland, and the UK. It has been saving lives for over 60 years.
From day 3, a few drops of blood are taken from the newborn's heel and spotted onto a special filter card (Guthrie card). The procedure takes seconds and causes only momentary discomfort. Timing is critical: before day 3, phenylalanine from protein in feeds may not yet be detectable at diagnostic levels.
The filter card is sent to a specialist screening laboratory. Tandem mass spectrometry (MS/MS) simultaneously measures phenylalanine alongside more than 25 other metabolic markers, allowing early detection of multiple inherited conditions from the same blood spot.
The lab compares the measured phenylalanine concentration against established reference ranges. Values above the cutoff trigger an automatic alert. Normal results require no follow-up action and are typically not actively communicated to parents in most countries — no news is good news.
If phenylalanine is elevated, parents are contacted promptly — usually by the maternity unit, the paediatrician, or directly by the screening programme. This is a preliminary finding, not a confirmed diagnosis. Parents are asked to bring the baby in as quickly as possible for a follow-up test.
A fresh blood sample is taken for a detailed confirmation test (quantitative plasma amino acid analysis). If PKU is confirmed, the baby is referred to a specialist metabolic centre within days. Treatment — a carefully calibrated low-phenylalanine diet — begins as soon as possible to protect brain development.
In Austria, mandatory newborn screening has been in place since 1966; in Germany since 1969; in Switzerland since 1965; in the UK since the 1970s. All costs are fully covered by the national health system.
"Positive" at the screening stage means the phenylalanine level was elevated. It is a signal for further testing — not a final diagnosis.
If the confirmation test establishes a persistently elevated phenylalanine concentration, the diagnosis is secured and the severity classified. Three main forms require different levels of treatment intensity:
Strictest dietary form. A very low Phe daily limit and lifelong amino acid formula (substitute) are required.
Dietary management is necessary, but the daily limit is somewhat more generous. BH4 (Sapropterin) therapy may be suitable.
Mildest form. Regular blood monitoring is required; strict dietary restriction is not always necessary at this level.
Important: These categories are a clinical guide, not a rigid system. Your child's individual treatment plan is set exclusively by the metabolic centre, taking into account genetics, blood values, age, and lifestyle.
Speed matters: the earlier treatment begins, the better the outcome for brain development. The first weeks are intensive — and then they become routine.
Once the diagnosis is confirmed, the baby is referred without delay to a specialist metabolic centre. These centres bring together paediatricians, dietitians, psychologists, and specialist nurses who focus on amino acid metabolism disorders. There are centres in all major cities across Austria, Germany, Switzerland, and the UK.
The low-phenylalanine diet starts immediately. For newborns this means: breast milk or standard infant formula is replaced by a precisely calculated combination of measured breast milk and a specialised amino acid formula. The formula is not a medication — it is a nutritional supplement providing all essential amino acids except phenylalanine, meeting the baby's full protein needs.
In the first weeks, blood phenylalanine is monitored very frequently — initially weekly or as directed by the team. The goal is to bring the Phe level into the target range quickly and keep it stable. As the child grows and values stabilise, monitoring intervals widen.
Tip: A directory of PKU metabolic centres in Austria, Germany, Switzerland, and the UK is available on this page.
For some patients with mild or moderate PKU, a medication called Sapropterin (BH4) can significantly raise the amount of phenylalanine the body can safely process.
Sapropterin (brand name: Kuvan) is a synthetic form of tetrahydrobiopterin (BH4) — a natural cofactor of the enzyme phenylalanine hydroxylase (PAH). In patients whose PAH enzyme has residual activity, BH4 can boost that activity enough to allow a substantially higher dietary Phe intake.
A BH4 responsiveness test is typically performed at the metabolic centre after the diagnosis is confirmed. The test involves measuring blood Phe levels before and after a supervised dose of Sapropterin over several hours or days. Only patients whose phenylalanine drops significantly in response are classified as "BH4-responsive" and may benefit from long-term therapy.
BH4 does not work for everyone — particularly not for patients with classic PKU caused by severely truncated PAH function. But for eligible patients, it is a meaningful therapeutic addition that can reduce dietary burden and improve quality of life. Your metabolic team will advise on whether testing is appropriate for your child.
All four countries operate well-established, universal newborn screening programmes. Key details differ slightly between systems.
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