The impact on quality of life of diet restrictions and disease symptoms associated with phenylketonuria: a time trade-off and discrete choice experiment study

Phenylketonuria (PKU) is a metabolic disorder leading to a deficiency in the metabolism of phenylalanine (Phe). Elevated Phe levels in the blood, tissue and brain may lead to emotional, cognitive, and physical symptoms in adults. To control blood Phe levels, most individuals with PKU need to follow a strict, life-long Phe-restricted diet. The main objective of this study was to estimate the impact of PKU-related disease symptoms and dietary restrictions on health-related quality of life (HRQoL). This study was designed as a web-based survey using the methods of Time Trade-Off (TTO) and Discrete Choice Experiment with duration (DCETTO) in a survey of the Swedish general population and population with PKU. Data were collected using questionnaires for TTO diet (n = 509), TTO symptoms (n = 507), and DCETTO (n = 1117). The disutility of diet restrictions ranged from 0.050 for a partially restricted diet without medical food to 0.193 for a fully restricted diet with medical food in the TTO (DCE: 0.043–0.108). The disutility of experiencing symptoms (emotional, cognitive and physical) ranged from 0.148 for mild symptoms to 0.593 for severe symptoms in the TTO (DCE: 0.122–1.522). The study shows that both diet and symptoms have a significant negative impact on HRQoL in PKU and that utility estimates are largely consistent across methods and samples.