The first European guidelines on phenylketonuria: Usefulness and implications for BH responsiveness testing.

Objective: This study aimed to investigate and improve the usefulness of the 48-hour BH loading test and to assess genotype for BH responsiveness prediction, using the new definition of BH responsiveness from the European guidelines, as well as an amended definition.

Method: Applying the definition of the European guidelines (≥100% increase in natural protein tolerance) and an amended definition (≥100% increase in natural protein tolerance or tolerating a safe natural protein intake) to a previous dataset, we first assessed the positive predictive value (PPV) of the 48-hour BH loading test using a cutoff value of 30%. Then, we tried to improve this PPV by using different cutoff values and separate time points. Last, using the BIOPKU database, we compared predicted BH responsiveness (according to genotype) and genotypic phenotype values (GPVs) in BH -responsive and BH -unresponsive patients.

Results: The PPV of the 48-hour loading test was 50.0% using the definition of the European guidelines, and 69.4% when applying the amended definition of BH responsiveness. Higher cutoff values led to a higher PPV, but resulted in an increase in false-negative tests. Parameters for genotype overlapped between BH -responsive and BH -unresponsive patients, although BH responsiveness was not observed in patients with a GPV below 2.4.

Conclusion: The 48-hour BH loading test is not as useful as previously considered and cannot be improved easily, whereas genotype seems mainly helpful in excluding BH responsiveness. Overall, the definition of BH responsiveness and BH responsiveness testing require further attention.