Phenylketonuria is an inborn error of protein metabolism.  Inborn errors of
protein metabolism involve a lack or great deficiency of the enzyme
necessary to metabolize one of the amino acids, in this case phenylalanine.
The inability of the normal metabolic route to be used, either fast enough
or at all (depending on the disorder), causes by-products to appear in
proportion to the amount of protein which one is attempting to metabolize.

The test which I am [sketchily] acquainted with measures the amount of a
specific by-product in the blood.  Naturally, the more protein one had
consumed, the more of the by-product that would be available to be
measured.  Therefore, the early testing which is done in the hospital (and
must be done, by state law in, as far as I know, all the states), would be
futile in a breastfed baby leaving soon after delivery.  Why would state
law require a test which is stressful, painful, and pretty much worthless?
Because the babies are a captive audience at that time.  The first test is
the only test many babies get (the babies whose mothers cannot afford well
baby care).  And also because the laws were written when longer stays were
the norm.

In Utah, we also test for galactosemia (an inborn error of carbohydrate
metabolism) and hypothyroidism at the same time.  Again, I would assume a
meal of lactose would be required for the test for galactosemia, but I am
unfamiliar with the test.  These three tests are the only ones performed in
Utah under the aegis, "PKU test."

Babies who have inherited PKU are born with normal intelligence,
demonstrating that they are protected from this build-up while in the womb.
At first blush then, using a sample of cord blood in the standard PKU test
seems not the best way to test for PKU.  [But I'm just guessing.]

Now, it is entirely possible that a laboratory test has been devised that
is either (a) more sensitive or (b) measures something different from the
standard PKU test, and if so, it could be that this test does not need the
baby to consume any amount of a protein-containing meal (such as colostrum)
before it is administered.  A test of cord blood was mentioned.  I asked my
husband if this were theoretically possible, and he said possibly someone
has developed or could develop a tissue culture using cord blood
lymphocytes which would be readable in 3 to 5 days, but I'm not sure it
would be feasible on a large scale.  This is just wild guessing on both our
parts.   It would be great to find out if there is indeed a better test for
PKU than the standard test, which was also economically feasible.

At one time PKU babies were born only to carriers (basically).  Now that we
can control the damaging effects of PKU through diet, we are starting to
have babies born to PKU mothers.  The severe dietary restrictions placed on
PKU children are eased after adulthood, but need to be reinstated during
pregnancy, a need which many of these women find repugnant.

There are a great many possible inborn errors of metabolism, besides those
mentioned.  Because they are even more rare than PKU, it is not economical
to produce a formula to fit the dietary needs for each type of inborn error
of metabolism.  Some babies die or are brain-damaged because of this.
According to Dr. Thomas Emery, some inborn errors of metabolism cease to be
problematic after puberty, a time of great change in the body.

Arly Helm




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