Alpha-Fetoprotein Testing
The alpha-fetoprotein (AFP) test
is a blood test done on the mother-to-be. As your baby grows inside
you, it produces alpha-fetoprotein in its liver. Some alpha-fetoprotein
crosses fetal membranes and enters your circulation. It’s possible to
measure AFP by drawing your blood; too much or too little
alpha-fetoprotein in your blood can be a sign of fetal problems.
Measurement of the amount of
alpha-fetoprotein in your blood can help your healthcare provider
predict problems in your baby, such as spina bifida or Down syndrome.
However, AFP detects only about 25% of Down syndrome cases. If Down
syndrome may be indicated, additional detailed diagnostic tests will
probably be ordered.
The AFP test is not done on all pregnant
women, although it is required in some states. It is not used routinely
in Canada. AFP is often used with other tests as
part of a multiple-marker test—a triple-screen test or a quad-screen
test; see the discussions below. If the test isn’t offered to you, ask
about it. There is relatively little risk, and it tells your doctor how
your fetus is growing and developing.
The AFP test is usually performed between
16 and 18 weeks of pregnancy. Timing is important and must be
correlated to the gestational age of your pregnancy and to your weight.
One important use of the test is to help a woman decide whether to have
amniocentesis.
If AFP detects a possible problem, more
definitive testing may be ordered. An elevated alpha-fetoprotein level
can indicate problems with the fetus, such as spina bifida or
anencephaly. An association has been found between a low level of
alpha-fetoprotein and Down syndrome. AFP can detect the following:
•neural-tube defects
•severe kidney or liver disease
•esophageal or intestinal blockage
•Down syndrome
•urinary obstruction
•osteogenesis imperfecta (fragility of the baby’s bones)
If your alpha-fetoprotein level is
abnormal, your doctor may choose to do a higher-level ultrasound,
amniocentesis or CVS to look for suspected problems. This ultrasound
may also help determine how far along in pregnancy you are.
One problem with the AFP test is a high
number of false-positive results; that is, the results say there is a
problem when there isn’t one. If 1000 women take the AFP test, 40 test
results come back as “abnormal.” Of those 40, only one or two women
actually have a problem.
If you have an AFP test and your
test result is abnormal, don’t panic. You’ll take another test and have
an ultrasound. Results from these second tests should give you a
clearer answer. Be sure you understand what “false-positive” and
“false-negative” test results mean. Ask your healthcare provider to
explain what each result can mean to you.
Multiple-Marker Tests
Tests that go beyond alpha-fetoprotein
testing are available to help your healthcare provider determine if
your child might have Down syndrome and to rule out other problems in
your pregnancy. They are called multiple-marker tests and include the triple-screen test and the quad-screen test.
The Triple-Screen Test
The triple-screen test helps
identify problems using three blood components—alpha-fetoprotein, human
chorionic gonadotropin (HCG) and a form of estrogen produced by the
placenta called unconjugated estriol.
Abnormal levels of these three blood
chemicals can indicate Down syndrome. For older mothers, the detection
rate is higher than 60%, with a false-positive rate of nearly 25%.
Abnormal results of a triple-screen test are usually double-checked
with ultrasound and amniocentesis.
The Quad-Screen Test
The quad-screen test is similar to the triple-screen but adds a fourth measurement—the blood level of inhibin-A,
a chemical produced by the ovaries and the placenta. This fourth
measurement raises the sensitivity of the standard triple-screen test
by 20% in determining if a fetus has Down syndrome. The quad-screen
test identifies almost 80% of fetuses with Down syndrome. It has a
false-positive rate of 5%.
