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Differential diagnosis of hyperphenylalaninaemia by a combined phenylalanine-tetrahydrobiopterin loading test.
[classical phenylketonuria]
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
(
BH
4
)
oral
loading
test
and
enables
the
selective
screening
of
BH
4
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
BH
4
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
8
h
after
administration
,
a
clear
response
was
obtained
with
the
higher
BH
4
dose
(
20
mg
/
kg
body
weight
)
,
allowing
detection
of
all
cases
of
BH
4
deficiency
,
as
well
as
differentiation
of
BH
4
synthesis
from
regeneration
defects
.
Since
DHPR
deficient
patients
who
were
previously
shown
to
be
non-responsive
to
the
simple
BH
4
loading
test
gave
a
positive
response
,
the
combined
Phe
plus
BH
4
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
.
Diseases
Validation
Diseases presenting
"as well as differentiation of bh4 synthesis from regeneration defects"
symptom
classical phenylketonuria
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