[tri-med] AFP and Maternal Serum Screens

This is a repost of my long and preachy post on AFP's and I have included
the values for the different trisomies at the end

Hi Romona and All,
My knowledge on AFP testing is really limited to that which is related to
genetic testing - so I may not be able to help you a great deal - but it may
provide a little background.

First AFP or Alphafetoprotein is a protein that we all have in our body
anyway. So if any of us were tested for AFP chances are it would be in the
normal range for our particular circumstances.

Secondly AFP is NOT a test for T-18 or any other problem during pregnancy.
IT IS A SCREENING TOOL ONLY. I cannot emphasise this enough. All that an AFP
can do is to indicate a degree of risk and alert a doctor to check further
for problems - the only definitive diagnosis for a trisomy during pregnancy
is a karyotype - and even then errors can be made or the results be
ambiguous. As someone has already mentioned AFP's have a very high false
negative or false positive rate.

Thirdly and AFP should never be used in isolation to diagnose a trisomy.
Usually it is part of a triple screen and includes HCG (human chorionic
gonadotropin) and UE3(unconjugated estriol) However to save money many
doctors start with an AFP and if that is outside the "normal" range in
pregnancy they will then do the other two. If all three of these indicate a
problem then the parents should be referred for genetic counselling and IF
THE PARENTS WANT FURTHER TESTING it should be offered along with
counselling. Further testing usually means either and ultrasound (detailed)
and or an amniocentesis.An ultrasound is also still only a screening tool.
And IMO no parent should panic based on AFP results alone - they can very,
very often be wrong for too many reasons.

Now for a little history - waaaaayy back when the dinosaurs roamed, OK in
1952 a bright spark doctor discovered AFP. Then in 1972 another doctor by
the name of Brock found that AFP was found in abnormally high concentrations
in the amniotic fluid of babies who had anencephaly (a type of neural tube
defect). A couple of years later they found that a high AFP was present in
neural tube defects in general.

10 years later they found that they could use the levels of of AFP to
predict Down Syndrome and then 3 or 4 years later they added T-18 to the
list and to a lesser extent T-13 but as far as I know it cannot be used to
predict any other trisomies or chromosomal anomalies. (talk to your
geneticist about that). It may just be that because the "other" trisomies
are rarer that no-one has enough numbers to draw the link.

Sooooo to be very clear - AFP testing was originally designed to detect
neural tube defects, the fact that it can (but not always) pick up other
chromosomal abnormalities is by chance it was not specifically designed to
detect T-18. I believe that AFP testing picks up somewhere between 50 and
90% of all T-18 pregnancies BUT each lab that does this testing makes their
own reference criteria - so the results (and their accuracy) are therefore
dependent on the number of tests they do. There are no universal reference
values. In Australia the absolutely best reference range is found in SA -
they have been keeping results of all AFP tests since the early 80's.

Who is offered AFP or maternal serum testing? Here in Australia if you go to
a private obstetrician then chances are you will be offered AFP testing
regardless of whether you are 18 or 80. If you go to a public hospital
clinic you probably will not be offered an AFP test unless you are 35 or
over. I believe that in the US all expecting mothers are offered AFP's -
mainly I think because doctors don't want to be sued for NOT offering it and
not picking up a problem.

Now how they calculate AFP levels and abnormal levels is EXTREMELY
complicated. It uses a very complex algorithm (I have a copy if anyone is
REALLY interested (but personally I think you would be nuts!! LOL). Other
factors that have significant bearing on the results are gestational age,
and the mothers weight. Similarly if there are twins that will greatly
effect the AFP results. The most common reason for incorrect risk
assessments in AFP is the gestational age - that is they are out in their
dates, secondly it is because the mother is expecting twins.

Using all these things the lab will calculate the increased risk for neural
tube defect, Down Syndrome, Trisomy 18 etc based on the MoM or median of the
mean. So the results do not say that you have a baby with T-18 but rather
that you have an increased risk of say 1:200 of having a child with T-18
over other pregnancies with the same factors. In very general terms an
abnormal AFP result that would indicate a pregnancy with a T-18 child the
AFP levels would be low. I do have a table here somewhere that I will post
later if I can find it that gives these "general" indicators for all the
maternal serum results (that is AFP, HCG and UE3).

Let me try and simplify that - say your risk of having a baby with T-18 is
1:100 and they do an AFP - the results of that AFP may bring that risk to
1:10 (high risk) or increase that risk to 1:1000 (low risk)

I mentioned that gestatational age plays an important factor - the optimal
time for conducting an AFP is between 16 and 18 weeks gestation as this is
the period when the AFP levels are at their peak of reliability. As the
pregnancy continues the levels of AFP will decrease.

Doctors like AFP's (or maternal serum screens) because they are minimally
invasive (blood test on the mother), relatively cheap (about $20 here in
Aus) and give them a better idea if there is something wrong early enough
for something to be done (i.e. termination of the pregnancy). Because an AFP
is or can be part of the "seek and destroy" mentality IMO the procedure
should be FULLY EXPLAINED to parents BEFORE it is done and the implications
of abnormal results discussed. Unfortunately many doctors see it now as
"routine" and it is only when abnormal results are found that parents know
why the test was done. But then again I don't even believe that there is
such a thing as a routine ultrasound these days.

Anyway, back to the thread on this - Tracey, yes AFP levels are found in the
amniotic fluid but usually the test is done via a blood draw. Levels in the
amniotic fluid are parallel to those in blood - but I believe the ratios are
a lot less.

The sugary drink someone mentioned is a glucose tolerence test and is used
to screen for gestational diabetes - it too is "routine" but is done a
little later in the pregnancy.

Now Romona,
There are a lot of different types of AFP - the one that I have been talking
about is maternal serum AFP (or MSAFP) and is specifically present in higher
concentrations in pregnancy. What you are talking about with the boys are
slightly different and it would seem from your post that they are looking
for increased levels of AFP as an indication of the presence of tumours.
Some tumours - particularly liver tumours produce elevated levels of AFP. I
know this in general terms but not in specifics - i.e. I know that they look
at AFP levels to indicate the presence of tumours, and I believe also polio
but that is all - sorry..............

I hope that helps...........


Remember - these tests are indicative of risk - NOT a diagnostic tool

T-21
AFP low
uE3 low
hCG high

T-13
AFP normal
uE3 no data
hCG low

T-18
AFP low
uE3 low
hCG low

Open Neural Tube Defect
AFP high
uE3 normal
hCG normal

IUGR, preterm birth, still birth
AFP high
uE3 no data
hCG no data

Multiple Gestation
AFP high
uE3 high
hCG high


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