The 48-hour tetrahydrobiopterin loading test in patients with phenylketonuria: Evaluation of protocol and influence of baseline phenylalanine concentration

Karen Anjema*, G. Venema, Floris C. Hofstede, E. C. Carbasius Weber, A. M. Bosch, Nienke M. ter Horst, Carla E. M. Hollak, C. F. Jonkers, M. E. Rubio-Gozalbo, E. M. C. van der Ploeg, M. C. de Vries, R. G. Janssen-Regelink, M. C. H. Janssen, H. Zweers-van Essen, Carolien C. A. Boelen, N. A. P. van der Herberg-van de Wetering, M. Rebecca Heiner-Fokkema, M. van Rijn, Francjan J. van Spronsen

*Corresponding author for this work

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Background: The 24- and 48-hour tetrahydrobiopterin (BH4) loading test (BLT) performed at a minimum baseline phenylalanine concentration of 400 mu mol/l is commonly used to test phenylketonuria patients for BH4 responsiveness. This study aimed to analyze differences between the 24- and 48-hour BLT and the necessity of the 400 mu mol/l minimum baseline phenylalanine concentration. Methods: Data on 186 phenylketonuria patients were collected. Patients were supplemented with phenylalanine if phenylalanine was = 30% reduction in phenylalanine concentration at >= 1 time point. Results: Eighty-six (46.2%) patients were responsive. Among responders 84% showed a >= 30% response at T = 48. Fifty-three percent had their maximal decrease at T = 48. Fourteen patients had >= 30% phenylalanine decrease not before T = 48. A >= 30% decrease was also seen in patients with phenylalanine concentrations
Original languageEnglish
Pages (from-to)S60-S63
JournalMolecular Genetics and Metabolism
Publication statusPublished - 2011


  • Phenylketonuria
  • PKU
  • Tetrahydrobiopterin
  • BH4
  • Sapropterin
  • Phenylalanine

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