Phenylalanine loading as a diagnostic test for DRD: interpreting the utility of the test.
Phenylalanine loading has been proposed as a diagnostic test for autosomal dominant DRD (dopa-responsive dystonia), and recently, a phenylalanine/tyrosine (phe/tyr) ratio of 7.5 after 4 h was reported as diagnostic of DRD. To test the utility of this test in another sample with DRD, we administered an oral challenge of phenylalanine (100 mg/kg) to 11 individuals with DRD and one non-manifesting gene carrier. Only 6/12 had a 4 h phe/tyr ratio of greater than 7.5, suggesting that additional parameters must be set to avoid missing the diagnosis of DRD, including the need for the plasma phenylalanine to reach a minimum level 600 in order for the test to be valid. We propose that in cases where this minimum plasma phenylalanine level is not reached, plasma tetrahydrobiopterin should be measured or alternatively other symptomatic family members should be screened.
Saunders-Pullman
R
R
Department of Neurology, Beth Israel Medical Center, New York, NY, USA. rsaunder@bethisraelny.org
Blau
N
N
Hyland
K
K
Zschocke
J
J
Nygaard
T
T
Raymond
D
D
Shanker
V
V
Mohrmann
K
K
Arnold
L
L
Tabbal
S
S
deLeon
D
D
Ford
B
B
Brin
M
M
Chouinard
S
S
Ozelius
L
L
Klein
C
C
Bressman
S B
SB
D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D004252:DNA Mutational Analysis; D017931:DNA Primers; D020821:Dystonic Disorders / Q000175:diagnosis*; D005069:Evaluation Studies as Topic; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D010375:Pedigree; D010649:Phenylalanine / Q000008:administration & dosage / Q000097:blood; D010651:Phenylalanine Hydroxylase / Q000235:genetics; D017422:Sequence Analysis, DNA; D013997:Time Factors; D014443:Tyrosine / Q000097:blood
D017931:DNA Primers; D014443:Tyrosine; D010649:Phenylalanine; D010651:Phenylalanine Hydroxylase