Tests during pregnancy:AFP screening test

How accurate is the AFP test?

AFP screening test and the Triple screening test
The AFP screening test is a measurement of alpha-fetoprotein. The Triple screening 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'.

Worth the worries?
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.

Making a decision means learning as much as possible about the possible outcomes, and thinking about what you would do in each case. Then ask: Will the results of the test (positive or negative) change what you plan to do? If not, then there is no reason to have the test and risk the anxiety that an abnormal result would bring. Why is the age of the fetus important?
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

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. What is apha-fetoprotein?
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. 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.



Comments: Tests during pregnancy - AFP-screening-test

Comments 1 to 4 of about 4.


emmah921 - 861 days ago.
just had a psitive result - got t u scan on tuesday 11thjan 2011 so will let u know x but read this has hellped

ellae - 1466 days ago.
I can honestly say that more often than not there are false positives. My AFP gave my son a 1 in 8 for downs. These tests can help pick up other issues. My Papp-a levels were very low, showing I had a placental issue, which was diagnosed after he was born. We also discoved that he did not have downs. So dont take your results for what they are worth until your lo is here!

ctwany29 - 1649 days ago.
Well i failed the test the first time so I thought I would fail it again. My chance was 1/500, but this time it is 1/23. My god that seems very high to me. I just had my son in may and he is perfect. I am due to have a little girl in april. I hope she comes out healthy like her brother. I am 32 years of age. I have had the ultrasounds and the doc says she looks fine so I hope she is.

misz-cashis-april09 - 1659 days ago.
i got my AFP like a week or 2 ago now. i dont even remember omg im soo bad with dates haha. but anywayy my dr didnt call me back and tell me anything. i guess i should call them and make sure. i hope theyd call me if anything was abnormal and i dont have another appt till nov 18th.hmm...

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