Urinary dihydroxanthopterin in the diagnosis of malignant hyperphenylalaninemia and phenylketonuria

Assessment of urinary dihydroxanthopterin is proposed as a simple method of recognition of patients with malignant hyperphenylalaninemia (MHPA). High levels of urinary dihydroxanthopterin are found in untreated patients with phenylketonuria (PKU) or with dihydropteridine reductase (DHPR) deficiency. After dietary control of the serum phenylalanine level in PKU, the urinary dihydroxanthopterin falls to near normal levels. In DHPR deficiency urinary dihydroxanthopterin levels are high even when serum phenylalanine levels are in the range achieved on dietary treatment. Low levels would be expected in patients with defects in tetrahydrobiopterin synthesis even before dietary treatment. Confirmation of the diagnosis of different forms of MHPA then requires more detailed studies, but dietary treatment of other PKU patients can proceed with confidence.