title: Oral phenylalanine loading test for the diagnosis of dominant guanosine triphosphate cyclohydrolase 1 deficiency.
abstract: OBJECTIVE
  To evaluate the usefulness of Phe loading test in patients for the diagnosis of guanosine triphosphate cyclohydrolase 1 deficiency (GTPCH).
  
  
  METHODS
  We studied one family composed of 13 members harbouring the Q89X mutation in the GTPCH gene, a non-related pediatric patient with GTPCH deficiency and 8 pediatric controls. 100 mg/kg of L-phenylalanine was orally administered, and blood spot samples were taken at baselines 1, 2, 4 and 6 h post-load.
  
  
  RESULTS
  Two out of 7 pediatric patients showed a phenylalanine/tyrosine ratio higher than the previously reported cut-off value of 5.25 at 4 h, while 6 of the 7 adult patients showed a higher value. The only adult patient with a phenylalanine/tyrosine ratio below 5.25 at 4 h was asymptomatic.
  
  
  CONCLUSIONS
  A cut-off value of 5.25 seems reliable for interpreting Phe loading test in adult patients with GTPCH deficiency, although a lower value should be established for pediatric patients.
authors.affiliation: Department of Paediatrics, Hospital Universitario Reina Sofía, Córdoba, Spain.
authors.forename: Eduardo
authors.initials: E
authors.lastname: López-Laso
authors.forename: Aida
authors.initials: A
authors.lastname: Ormazabal
authors.forename: Rafael
authors.initials: R
authors.lastname: Camino
authors.forename: Francisco J
authors.initials: FJ
authors.lastname: Gascón
authors.forename: Juan J
authors.initials: JJ
authors.lastname: Ochoa
authors.forename: María E
authors.initials: ME
authors.lastname: Mateos
authors.forename: María J
authors.initials: MJ
authors.lastname: Muñoz
authors.forename: Juan L
authors.initials: JL
authors.lastname: Pérez-Navero
authors.forename: José I
authors.initials: JI
authors.lastname: Lao
authors.forename: María A
authors.initials: MA
authors.lastname: Vilaseca
authors.forename: Rafael
authors.initials: R
authors.lastname: Artuch
mesh_terms: D000284:Administration, Oral; D000328:Adult; D000592:Amino Acid Metabolism, Inborn Errors / Q000175:diagnosis*; D002648:Child; D002675:Child, Preschool; D005260:Female; D006136:GTP Cyclohydrolase / Q000172:deficiency*; D006801:Humans; D007223:Infant; D008297:Male; D008875:Middle Aged; D010649:Phenylalanine / Q000008:administration & dosage*; D013997:Time Factors
chemical_list: D010649:Phenylalanine; D006136:GTP Cyclohydrolase