Blood Phenylalanine Control in Paediatric and Adult Centres in the UK: Data from 2012-2018

Background: Metabolic control in phenylketonuria (PKU) is known to deteriorate with age, but national-level data describing blood phenylalanine (Phe) control across the United Kingdom (UK) are limited. Objective: To characterise blood Phe control in individuals with PKU attending UK metabolic centres. Methods: Sixteen UK centres (nine paediatric, six adult, one mixed) retrospectively extracted blood Phe results collected between 2012 and 2018. Demographic, phenotypic and monitoring-related variables were analysed. Written consent for data collection was obtained from all patients or their caregivers. Results: Data were available for 871 individuals (55% female), of whom 744 (85%) were classified as follows: classical PKU, 75%, mild PKU, 22% and hyperphenylalaninaemia, 3%. Mean blood Phe concentrations were significantly higher in adults than children (491 ± 308 vs. 303 ± 199 µmol/L; p < 0.001), and the proportion of samples within target range declined steadily with age, from 78% in children < 2 years to 36% in adults ≥ 41 years. Individuals with classical PKU had higher mean Phe concentrations and lower target attainment than those with HPA (386 vs. 300 µmol/L; 61% vs. 78%; p < 0.001), while mild PKU and HPA showed comparable control. Females generally demonstrated better metabolic control than males. More frequent dried blood spot sampling for blood Phe was strongly associated with improved metabolic control: weekly (254 ± 175 µmol/L; 82% within target), fortnightly (319 ± 207 µmol/L; 70%), monthly (397 ± 231 µmol/L; 61%), and less than monthly (624 ± 349 µmol/L; 44%). Nearly half of the blood Phe samples (47%) with recorded timing were taken in a non-fasting state. Conclusions: Achieving lifelong metabolic stability on a Phe-restricted diet alone remains challenging. These national data highlight the need for broader therapeutic options to support individuals with PKU across the lifespan.