Oral phenylalanine loading test for the diagnosis of dominant guanosine triphosphate cyclohydrolase 1 deficiency.
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.
López-Laso
Eduardo
E
Department of Paediatrics, Hospital Universitario Reina Sofía, Córdoba, Spain.
Ormazabal
Aida
A
Camino
Rafael
R
Gascón
Francisco J
FJ
Ochoa
Juan J
JJ
Mateos
María E
ME
Muñoz
María J
MJ
Pérez-Navero
Juan L
JL
Lao
José I
JI
Vilaseca
María A
MA
Artuch
Rafael
R
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
D010649:Phenylalanine; D006136:GTP Cyclohydrolase