Newborn screening for Phenylketonuria enables early detection and timely treatment with a phenylalanine-restricted diet to prevent severe neurological impairment. Although effective and in use for 60 years, screening, diagnostic, and treatment practices still vary widely across countries and centers. To evaluate the Phenylketonuria newborn screening practices internationally, we designed a survey with questions focusing on the laboratory aspect of the screening system. We analyzed 24 completed surveys from 23 countries. Most participants used the same sampling age range of 48-72 h; they used tandem mass spectrometry and commercial non-derivatized kits to measure phenylalanine (Phe), and had non-negative cut-off values (COV) set mostly at 120 µmol/L of Phe. Participants mostly used genetic analysis of blood and detailed amino acid analysis from blood plasma as their confirmatory methods and set the COV for the initiation of dietary therapy at 360 µmol/L of Phe. There were striking differences in practice as well. While most participants reported a 48-72 h range for age at sampling, that range was overall quite diverse Screening COV varied as well. Additional screening parameters, e.g., the phenylalanine/tyrosine ratio were used by some participants to determine the screening result. Some participants included testing for tetrahydrobiopterin deficiency, or galactosemia in their diagnostic process. Results together showed that there is room to select a best practice from the many practices applied. Such a best practice of PKU-NBS parameters and post-screening parameters could then serve as a generally applicable guideline.

Center for Inherited Metabolic Diseases Karolinska University Hospital SE 171 76 Stockholm Sweden

Center for Rare Diseases University Children's Hospital Ljubljana University Medical Center Bohoričeva 20 1000 Ljubljana Slovenia

Clinical Institute for Special Laboratory Diagnostics University Children's Hospital Ljubljana University Medical Center Vrazov trg 1 1000 Ljubljana Slovenia

Department of Endocrinology Diabetes and Metabolic Diseases University Children's Hospital Ljubljana University Medical Center Bohoriceva 20 1000 Ljubljana Slovenia

Department of Molecular Medicine and Surgery Karolinska Institutet SE 171 76 Stockholm Sweden

Department of Pediatrics and Inherited Metabolic Disorders Charles University 1st Faculty of Medicine and General University Hospital Prague Ke Karlovu 2 128 08 Praha 2 Czech Republic

Division of Metabolic Diseases Beatrix Children's Hospital University Medical Center Groningen University of Groningen 9718 GZ Groningen The Netherlands

Faculty of Medicine University of Ljubljana Vrazov trg 2 1000 Ljubljana Slovenia

INSERM UMRS 1256 Nutrition Genetics and Environmental Risk Exposure Faculty of Medicine of Nancy University of Lorraine 54505 Nancy France

International Society for Neonatal Screening Reigerskamp 273 3607 HP Stichtse Vecht The Netherlands

Norwegian National Unit for Newborn Screening Division of Pediatric and Adolescent Medicine Oslo University Hospital 0424 Oslo Norway

Pediatric Unit Reference Center for Inborn Errors of Metabolism University Hospital of Nancy 54500 Nancy France

Regional Coordinator Centre for Rare Diseases University Hospital of Udine 33100 Udine Italy

Sheffield Children's FT Sheffield S10 2TH UK

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