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 <400 mu mol/l. BH4 20 mg/kg was administered at T = 0 and T = 24. Blood samples were taken at T = 0, 8, 16,24 and 48 h. Responsiveness was defined as >= 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 <400 mu mol/l. Conclusion: In the 48-hour BLT, T = 48 seems more informative than T = 24. Sampling at T = 32, and T = 40 may have additional value. BH4 responsiveness can also be predicted with baseline blood phenylalanine <400 mu mol/l, when the BLT is positive. Therefore, if these results are confirmed by data on long-term BH4 responsiveness, we advise to first perform a BLT without phenylalanine loading and re-test at higher phenylalanine concentrations when no response is seen. Most likely, the 48-hour BLT is a good indicator for BH4 responsiveness, but comparison with long term responsiveness is necessary. (C) 2011 Elsevier Inc. All rights reserved.