The first European guidelines on phenylketonuria: Usefulness and implications for BH 4 responsiveness testing

Objective This study aimed to investigate and improve the usefulness of the 48‐hour BH 4 loading test and to assess genotype for BH 4 responsiveness prediction, using the new definition of BH 4 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 4 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 4 responsiveness (according to genotype) and genotypic phenotype values (GPVs) in BH 4 ‐responsive and BH 4 ‐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 4 responsiveness. Higher cutoff values led to a higher PPV, but resulted in an increase in false‐negative tests. Parameters for genotype overlapped between BH 4 ‐responsive and BH 4 ‐unresponsive patients, although BH 4 responsiveness was not observed in patients with a GPV below 2.4. Conclusion The 48‐hour BH 4 loading test is not as useful as previously considered and cannot be improved easily, whereas genotype seems mainly helpful in excluding BH 4 responsiveness. Overall, the definition of BH 4 responsiveness and BH 4 responsiveness testing require further attention.