Differential diagnosis of hyperphenylalaninaemia by a combined phenylalanine-tetrahydrobiopterin loading test

A. Ponzone, O. Guardamagna, M. Spada, S. Ferraris, R. Ponzone, L. Kierat, N. Blau

Research output: Contribution to journalArticlepeer-review


We describe a new fully reliable method for the differential diagnosis of tetrahydrobiopterin-dependent hyperphenylalaninaemia (HPA). The method comprises the combined phenylalanine (Phe) plus tetrahydrobiopterin (BH4) oral loading test and enables the selective screening of BH4 deficiency when pterin analysis is not available or when a clear diagnosis has not been previously made. It should be performed together with the measurement of dihydropteridine reductase (DHPR) activity in blood. The new combined loading test was performed in nine patients with primary HPA, three with classical phenylketonuria (PKU), three with DHPR deficiency, and three with 6-pyruvoyl tetrahydropterin synthase (PTPS) deficiency. Three hours after oral Phe loading (100 mg/kg body weight), synthetic BH4 was administered orally at doses of either 7.5 or 20 mg/kg body weight. Amino acid (Phe and tyrosine) and pterin (neopterin and biopterin) metabolism and kinetics were analysed. By exploiting the decrease in serum Phe 4 and 8h after administration, a clear response was obtained with the higher BH4 dose (20 mg/kg body weight), allowing detection of all cases of BH4 deficiency, as well as differentiation of BH4 synthesis from regeneration defects. Since DHPR deficient patients who were previously shown to be non-responsive to the simple BH4 loading test gave a positive response, the combined Phe plus BH4 loading test can be used as a more reliable tool for the differential diagnosis of HPA in these patients. Moreover, it takes advantage of being performed while patients are on a Phe-restricted diet.

Original languageEnglish
Pages (from-to)655-661
Number of pages7
JournalEuropean Journal of Pediatrics
Issue number8
Publication statusPublished - Aug 1993


  • Hyperphenylalaninaemia
  • Loading test
  • Neonatal screening
  • Phenylketonuria
  • Tetrahydrobiopterin deficiency

ASJC Scopus subject areas

  • Pediatrics, Perinatology, and Child Health


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