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misc.kids FAQ on the Pregnancy AFP Screen and the Triple Screen



 
 
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Old May 21st 06, 05:22 AM posted to misc.kids.info,misc.answers,news.answers,misc.kids.pregnancy
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Default misc.kids FAQ on the Pregnancy AFP Screen and the Triple Screen

Archive-name: misc-kids/pregnancy/screening/AFP
Posting-Frequency: monthly
Last-Modified: January 4, 1999

------------------------------------------------------------

Misc.kids Frequently Asked Questions

AFP Screen and the Triple Screen

================================================== ===================
Collection originally written by Dorothy Neville
) and currently maintained by Lynn Gazis-Sax
).
Last updated: 1/4/99

To contribute to this collection, please send e-mail to the address given
above, and ask me to add your comments to the FAQ file on AFP Screen and
the Triple Screen. Please try to be as concise as possible, as these FAQ
files tend to be quite long as it is. And, unless otherwise requested, your
name and e-mail address will remain in the file, so that interested readers
may follow-up directly for more information/discussion.

For a list of other FAQ topics, ftp to the pub/usenet/misc.kids directory
of rtfm.mit.edu or tune in to misc.kids.info.
=====
Copyright 1994, Dorothy Neville. Use and copying of this information are
permitted as long as (1) no fees or compensation are charged for use,
copies or access to this information, and (2) this copyright notice is
included intact.
====

================================================== ===================
[NOTE: this is information collected from many sources and while I have
strived to be accurate and complete, I cannot guarantee that I have
succeeded. This is not medical advice. For that, see your doctor or other
health care provider.]

SUMMARY:

These screening tests are used to predict which pregnancies are at elevated
risk of certain abnormalities. They do not give conclusive results; an
abnormal result means that more testing is suggested. This can be very
distressing. Especially distressing is that some caregivers and books do
not make clear the inconclusive nature of the tests; women have been told
that their baby definitely has a defect on the basis of this test. This is
irresponsible medicine. Most women with abnormal results will have healthy
normal babies. Below are some details about the tests and what they can and
cannot do. The information is mostly taken from two brochures prepared by
the Laboratory of Pathology of Seattle and the Swedish Hospital Medical
Center. I have also added information from other sources.

Every woman contemplating these tests should have access to accurate
information in order to make an informed decision about whether or not to
have the tests, and how to interpret the results. Unfortunately there are
enough stories on misc.kids to show that this does not happen. Perhaps the
following will be helpful to those contemplating having the screen, or
people who have had the screen and have gotten disturbing results.

INDEX:

1. What are the Screens?

Neural tube defects:

2.a What is alpha-fetoprotein
2.b Why is age of the fetus important?
2.c What are the neural tube defects and their severity?
2.d What are my chances of having a baby with neural tube defects?
2.e What about folic acid?
2.f Will this test detect all cases of neural tube defects?

Down's Syndrome

3.a What is Down's syndrome?
3.b Will this test detect all cases of Down's?

General

4.a Will this test guarantee a normal baby?
4.b How are the results presented?

5.a What should I do if the values are abnormal?
5.b If the ultrasound is normal, does that guarantee a normal baby?

6.a What are the benefits of the test?
6.b What are the risks of the test?
6.c Should I have the screen?

7. What are some resources for more information?

8. Some stories from women who have had the tests.
----------------------------------------------------

1. What are the screens?

1. This FAQ covers questions about two similar tests that are available for
pregnant women. The first, AFP, is a measurement of alpha-fetoprotein. The
second test is newer and combines the AFP test with measurements of two
hormones, unconjugated estriol and human chorionic gonadotropin. It is
often called the "triple screen".
-----------------------------

Addition by Dr. Tim Reynolds:

The triple test is a mis-nomer. The test combines AFP with a variety of
different assays and no definite proof of which is best currently exists In
fact they are all about the same so far as data available allows us to
tell. Combinations available a

AFP + total HCG

AFP + Free beta HCG (free beta is a subsection of the HCG molecule)

AFP + total HCG + unconjugated estriol (uE3)

AFP + free beta HCG + uE3

AFP + free beta HCG + uE3 + Free alpha HCG

AFP + free beta HCG + uE3 + neutrophil alkaline phosphatase

The detection rates reported for these different combinations all suggest
Detection rate is about 60% and differences of 1-2% exist between different
combinations: However the data available is not sufficient to determine
which combination is better. Other evidence suggests that as the number of
analytes used to estimate risk increases, the errors become multiplied and
the result becomes less accurate. It is probably better therefore to use
the simpler screening combinations (AFP + total or free beta HCG) and not
bother with uE3, free alpha or any of the other markers. If you are told
that you can have a 2-marker test or a 3-marker test if you pay mo opt
for the 2-marker test.
-----------------------------
Both tests use a blood sample from the mother at a specific time during
pregnancy. The AFP can be done during weeks 16 to 18 LMP, the triple screen
can be done from weeks 15 to 18 LMP.
-----------------------------

Addition by Dr. Tim Reynolds:

The test can be done using AFP + free beta from 11 weeks, but the
diagnostic test has a very high rate of miscarriage (up to 5%) so it is
best to wait until 14 weeks (at least). The accuracy of the test is
affected by gestation dating: It is best to have an ultrasound scan before
the screen to confirm your gestation dates.
-----------------------------
An elevated level of AFP in the sample indicates an increased risk of
neural tube defects in the fetus. A depressed level of AFP indicates an
increased risk of Down's syndrome. The triple screen uses levels of all
three substances to indicate increased risk of Down's. This is considered a
more accurate screen for Down's.
-----------------------------

Addition by Dr. Tim Reynolds:

Down's syndrome is associated with decreased AFP and uE3, and increased
HCG. A combination of low AFP and high HCG is particularly significant. The
maths of calculating results is quite complex but also takes into account
the maternal age.
-----------------------------
Neither test can tell you if something is wrong with your baby. Only a
diagnostic test (such as chorion villi sampling or amniocentesis) can tell
definitively if something is wrong. The AFP and the triple screen help to
identify pregnancies that are at increased risk.

BEING AT INCREASED RISK DOES NOT AUTOMATICALLY MEAN YOUR BABY HAS A DEFECT!
Most women who find themselves in the elevated risk category give birth to
healthy, normal children. This fact is often missed when interpreting the
results of this screen.
-----------------------------

Addition by Dr. Tim Reynolds:

In young women, the predictive value of a +ve Downs screen result is about
1%: i.e. only 1 in every 100 young women with a +ve result will be carrying
a Down's fetus. In older women, the +ve pred. value is about 4%.
-----------------------------

2.a What is apha-fetoprotein?

2.a AFP is a protein produced by every fetus. The baby urinates it into the
amniotic fluid, and it makes its way from there into the mothers blood for
excretion. There is a normal range of AFP found in the mothers blood; this
is determined by the age of the fetus.

A baby with an open spinal defect often leaks larger quantities of AFP into
the amniotic fluid, and in turn, into the mothers bloodstream.

An elevated level of AFP *does NOT* diagnose a neural tube defect! Babies
with such defects tend to produce more AFP, which gets into the mother's
blood, but there are also other reasons for an elevated level. The most
common reason for elevated AFP is that the age of the fetus was estimated
incorrectly. Twin pregnancy would also tend to show elevated levels.
-----------------------------

Addition by Dr. Tim Reynolds:

This is a good reson why an ultrasound should be done first because it is
not possible to do Down's screens on multiple pregnancies: An amnio is
needed for confirmation of result and it is virtually impossible to be
certain that trisomy cells found in a twin pregnancy come from a particular
twin: This leads to the possibility of a selective abortion in which the
wrong twin is aborted: Ergo, Down's screening in twin pregnancies is very
inadvisable.
-----------------------------

2.b Why is the age of the fetus important?

2.b The normal ranges of AFP and the hormones of the triple screen will
vary with the age of the fetus. So in order to determine if the values are
in the normal range, one must have an accurate date. This can be
problematic if date of conception or last menstrual period is not known.
For women with cycles significantly longer or shorter than 28 days, or who
don't have regular cycles at all, scheduling or interpreting the results of
the screens will be difficult if not impossible without ultrasound to more
accurately date the fetus.

The first thing that many doctors do when a patient has an abnormal AFP or
triple screen is request an ultrasound to recalculate the age of the fetus
(and to rule out twins). Often the recalculated age is enough evidence to
rule the screen result normal.
-----------------------------

Addition by Dr. Tim Reynolds:

Better to do scan first (this is standard practice in most centres in the
UK).
-----------------------------

2.c. What are the neural tube defects and their severity?

2.c The two main defects are spina bifida (open spine) and anencephaly.

The problems associated with spina bifida will vary with the size, location
and nature of the opening in the spine. Problems range from mild backaches
to severe mental retardation, paralysis, bowel and bladder control problems
and leg deformities. Generally speaking, the AFP test is more likely to
detect the more severe forms of spina bifida.

Anencephaly is a failure of the brain and skull to fully develop. It is
incompatible with life.

2.d What are my chances of having a baby with a neural tube defect?

2.d In the US, about 1,600 to 2,000 babies are born with spina bifida each
year. About 800 are born with anencephaly. (from American Baby, jul 93)

The cause(s) are unknown. Having one child with a neural tube defect does
increase the probability of having another. Yet, over 95% of babies with
neural tube defects are born to families in which there is no history of
birth defects or pregnancy problems.

2.e What about Folic Acid?

2.e Folic acid (or folacin) is a vitamin found in green vegetables, legumes
and other sources. An average diet is generally deficient in folic acid.
Studies have shown that when women with known risk factors of having a baby
with neural tube defects take folic acid supplements from before conception
to about 8 weeks of pregnancy, they can reduce their risk for these
defects. A more recent study claims that this is beneficial for all women,
not just those with previous risk factors.

For pregnant women or those trying to get pregnant, the recommended daily
amount of folic acid is either 400 or 800 micrograms (.4 or .8 mgs)
depending on what source you read. It is very unwise to take more than a
1000 micrograms (1 mg) a day, as folic acid at this level can mask other
serious health problems. In fact, because of this caution, in some places
folic acid is not sold over the counter as a stand alone supplement. This
may be changing due to this new link to neural tube defects. I was able to
find 800 mcg folic acid capsules over the counter in Washington state.
Someone from Pennsylvania reports that it is not available there except by
prescription. All prenatal vitamins ought to have at least 400 mcg. The FDA
is also considering requiring fortification of grain products with folacin.

While the benefit of taking folic acid on neural tube defects is limited to
early in pregnancy, it is still an important nutrient for building red
blood cells and is important throughout the entire pregnancy. Some doctors
are beginning to consider it as important as iron supplementation. Talk to
your dr for his or her recommendation.

2.f Will this test detect all cases of neural tube defects?

2.f No. The screening process will detect 80% of the cases of spina bifida
and 90% of the cases of anencephaly.
-----------------------------

Addition by Dr. Tim Reynolds:

Hence the argument for a detailed ultrasound scan at 18 weeks (routine UK
practice in most good centres is a dating scan either at booking (8-12
weeks), or when the Down's test is collected (16 weeks): PLUS, a detailed
'anomaly scan' at 18-20 weeks.
-----------------------------

3.a What is Down's syndrome?

3.a This chromosomal disorder is characterized by varying degrees of mental
retardation and an increased risk of physical defects. It occurs in about
one in six hundred births. The risk increases with the age of the mother,
but women of any age can give birth to a baby with Down's. Until recently
it was thought that a woman's age was the only indicator of Down's risk.
Measuring the amount of AFP, or better, the triple screen, and using this
information as well as age will give a more accurate estimate of the risk
of delivering a baby with Down's.

3.b Will this test detect all cases of Down's?

No. 25% of the cases of Down's will result in abnormal AFP, while 60% of
the cases will register as an abnormal triple screen.
-----------------------------

Addition by Dr. Tim Reynolds:

Above line incomplete:

The detection rate is dependant on maternal age: in young women (age
12-16), the average detection for a multi-marker test will be about 40-45%
and for an AFP only test, about 20% (There is NO SANE REASON for testing
for Down's using AFP only: this is completely superceeded by multimarker
testing). In older women the detection rate for the Down's screen is much
higher and can be up to 90-95% in women aged 45+. The 60% figure quoted in
most women's magazines is the average detection rate for a normal
population assuming all women are screened. If all women over 35 get an
amnio and only younger women are screened, the apparent success of the
screening test will look poorer: If only older women get screened, the
apparent success rate will look better (Many of the differences in
detection rates for different screening combinations can at least partially
be explained by age distribution differences in the study populations).
-----------------------------

4.a Will this test guarantee a normal baby?

No. Most defects are not detected by this or any other test. However a few
rare defects may be detected in addition to neural tube defects and Down's.
Some pregnancies which may be at increased risk for low birth weight and
prematurity are occasionally identified, as are some twin pregnancies. Note
that this identification does not come from the screen itself, but from
follow-up testing.

4.b How are the results presented?

The results may be presented in a number of ways. It is important to
understand what your numbers mean. Ask your caregiver to explain.

The triple screen might be given as a probability such as: "based on
maternal age your risk of Down's is 1/390 --- when the levels of AFP, HCG
and uE3 are also taken into account, your risk is 1/14000. Based on AFP
levels, your risk of neural tube defects is 1/1400."

Or the results might simply be positive/negative, or normal/abnormal. You
will probably want to know exactly what that means. Ask. In fact, there is
controversy as to what "abnormal" means. Try to find out what your lab
considers abnormal. They are working from the more detailed probability
information.

I think that it is becoming more common for the results to be given as
probabilities, which are more meaningful than a simple "positive/negative"
which is impossible to interpret. Probabilities can still be kind of scary
to interpret though.
-----------------------------

Addition by Dr. Tim Reynolds:

Probabilities can be very misleading and are often 'over-interpreted by
obstetricians as a way of avoiding doing amniocenteses. The usual risk cut
off in the UK is 1 in 300: Using this cut off about 1 in every 50 amnios
will yeild an abnormality. Some obstetricians use 1 in 150 as a cut off.
This halves the number of amnios they do. The rationale for using this cut
off is that the risk of abortion due to amnio is about 1 in 150. Since in
the 1 in 300 group you expect 1 in 50 amnios to give an abnormal result it
is evident that their argument is specious.
-----------------------------

5.a What should I do if the values are abnormal?

Your caregiver should recommend an ultrasound to get a more accurate dating
and to rule out multiple fetuses. Over 90% of the time, abnormal AFP values
are due to these or other benign factors. Then a second AFP test might be
run. If that is also abnormal, a more sophisticated ultrasound and
amniocentesis would be suggested. The diagnosis of neural tube defects
usually requires a combination of AFP testing, ultrasound and
amniocentesis. Diagnosing Down's requires amniocentesis, but an ultrasound
can be suggestive (see next question).
-----------------------------

Addition by Dr. Tim Reynolds:

It is best to get amnio first as this prevents the anxiety!
-----------------------------
Since time and worry can be an issue, perhaps you and your caregiver will
decide to get the more sophisticated level 2 ultrasound on the basis of one
abnormal test. This is something that you should discuss with your
caregiver.

5.b If my ultrasound is normal, does that guarantee a normal baby?

5.b No. The ultrasound exam, however. will identify the majority of defects
which cause a high AFP level. Factors such as the experience of the
ultrasonographer, the weight of the mother, the position of the fetus and
the quality of the equipment can affect its accuracy. In addition, there
are no reliable ultrasound findings for Down's.

However, Down's can be strongly suggested by an ultrasound because Down's
babies often have a certain body/face type. From one woman:

"There are, however, several characteristics that are common to Downs'
babies that a skilled ultrasound technician can look for. My technician
measured the femur (usually short in Downs' Syndrome babies), checked the
number of blood vessels in the umbilical cord (Downs' babies sometimes have
one fewer), checked the width of the forehead (usually wider for a Downs'
baby), and checked the heart (apparently, Downs' babies are more likely to
suffer from heart defects). A finding that none of these characteristics
are present does not guarantee a healthy baby, but it may provide some
reassurance, particularly in a case where the AFP reading is only
borderline low."

Here's some statistics about elevated AFP and normal ultrasounds:

Physician-researchers at Boston's Brigham and Women's Hospital,
after studying 87,584 pregnancies, reported that an elevated AFP
[alpha-feto-protein] level in conjunction with a normal
ultrasound scan implies a less than 0.1 percent chance that the
baby will have one of the four most common birth defects. In
light of the finding that amniocentesis itself carries a 0.5 to
1.5 percent chance of terminating a pregnancy (Robin J.R. Blatt,
Prenatal Tests), the researchers have concluded that "many women
may choose not to have an amniocentesis when informed that the
risk of pregnancy loss is substantially greater than the
likelihood of finding an anomaly". (New England Journal of
Medicine 323, No. 9, Aug 30, 1990)

6.a What are the benefits of the test?

It can help you make decisions about your pregnancy. If the results are
abnormal, and follow-up diagnostic testing shows defects, some women may
decide to terminate the pregnancy. For those who carry an affected baby to
term, the information can help in managing the delivery and early care of
the infant. This can improve the long and short term outlook for the child.
Sometimes when the AFP level is high, but the baby is normal, it is a
forewarning of later pregnancy complications such as prematurity or low
birthweight. Knowledge of this risk may alter the management of your
pregnancy.

There is recent evidence that shows that babies born with spina bifida may
benefit greatly from being delivered via cesarean section. So having a
cesarean in addition to other early intervention could significantly change
the outcome for such a child and his/her family. (the woman who told me
this could not find the source for reference. Your doctor might have more
information. I could not find anything on medline but I am a medline
novice.) [It looks equivocal. There is no consensus on this, and it may end
up being decided one way or the other--so the answer is, ask the doctor.]

6.b What are the risks of the test?

An abnormal result may cause considerable worry and concern. Since most
women with abnormal results will have healthy normal babies, you may decide
that the test is not worth the possible anxiety. Some statistics: about 5%
of women tested will have abnormal readings. About 90% of those will not
have affected babies, but have abnormal values because the dates were
calculated wrong, there are twins, or other reasons. So, for every 1000
women tested, about 50 will be told they have increased risk, and of the
50, about 45 or more will in fact have normal pregnancies. Some people feel
that the high level of "false-positive" readings make the test not worth
the risk.

The tests are relatively cheap. However, follow-up diagnostic testing is
not. These costs may be covered by insurance.

The test itself has no risk to the baby. The only risk to the mom is pain
from the blood draw. However, if the results are abnormal, you may wish to
opt for an amnio to relieve your worries. Amnio does carry a small risk of
miscarriage.

The test will not identify all cases of neural tube defects or Down's. The
problems for a baby with spina bifida range from the very minor to the very
severe. Generally speaking, the test detects only the more severe problems.
However, you may not be able to get exact details about the severity in any
particular case. This could lead to months of anxiety.

The AFP cannot be done until week 15 or 16, and takes about a week to get
the results. Followup amnio means even more waiting, so if termination is
decided, it will probably not happen until week 18 at the earliest. This
can be very difficult emotionally, and more difficult physically than an
early termination option with the CVS.
-----------------------------

Addition by Dr. Tim Reynolds:

Depending on the equipment