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

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