PFSI008
How the Test Is Done
CELL-FREE DNA PRENATAL
SCREENING TEST
What is it?
The cell-free DNA
prenatal screening test (also
called “cfDNA test”) screens for
certain conditions caused by an
abnormal number of
chromosomes. It does not test
for all types
of chromosomal
disorders.
Screening tests are used to
estimate whether your baby is
at higher risk or lower risk of
having a certain condition.
Diagnostic tests can give
a definite answer about
whether the baby has a certain
condition. These tests include
amniocentesis or chorionic
villus sampling (CVS).
= maternal DNA
= DNA from the pregnancy
PFSI008: Designed as an aid to patients, this document sets forth current information and opinions related to women’s health. The information
does not dictate an exclusive course of treatment or procedure to be followed and should not be construed as excluding other acceptable methods
of practice. Variations, taking into account the needs of the individual patient, resources, and limitations unique to the institution or type of
practice, may be appropriate.
Copyright September 2015 by the American College of Obstetricians and Gynecologists. No part of this publication may be reproduced, stored
in a retrieval system, posted on the Internet, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or
otherwise, without prior written permission from the publisher.
A blood sample is
taken from the pregnant
woman that contains
maternal DNA and DNA
from the pregnancy.
The sample is analyzed
in a laboratory to check
for the presence of an
abnormal amount of DNA
from chromosomes 21,
18, and 13.
Major conditions
screened for:
• Trisomy* 21 (Down syndrome)
• Trisomy 18
• Trisomy 13
Conditions not
screened for:
• Problems that are screened for
by ultrasound, such as neural
tube defects, heart defects, and
abdominal wall defects
• Many other chromosomal and
genetic disorders
How is it done?
Some of the genetic
material (DNA) from the
pregnancy circulates
in the mother’s
bloodstream. The cfDNA test
is done on a sample of
the pregnant woman's
blood.
When can it
be done?
It can be done as early
as 9–10 weeks of
pregnancy and up
until delivery.
*Trisomy means that there are three copies of a particular chromosome instead of the normal two copies.
For instance, trisomy 21 means that there are three copies of chromosome 21.
→
Sometimes the test
does not yield a result
or is indeterminate. In
this case, you should
receive further genetic
counseling, and an
ultrasound exam or
diagnostic testing
should be offered due
to an increased risk
of a chromosomal
disorder.
PFSI008: Designed as an aid to patients, this document sets forth current information and opinions related to women’s health. The information
does not dictate an exclusive course of treatment or procedure to be followed and should not be construed as excluding other acceptable methods
of practice. Variations, taking into account the needs of the individual patient, resources, and limitations unique to the institution or type of practice,
may be appropriate.
Copyright September 2015 by the American College of Obstetricians and Gynecologists. No part of this publication may be reproduced, stored in a
retrieval system, posted on the Internet, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise,
without prior written permission from the publisher.
What do the
results mean?
If you get a positive result, how
likely is it that the baby actually has
the disorder? This is called the positive
predictive value (PPV) of the test.
If you get a negative result, how likely
is it that the baby actually does not
have the disorder? This is called
the negative predictive value
(NPV) of the test.
For Example: Trisomy 21 (Down Syndrome)
For the cfDNA
test, the positive
predictive value
depends on how frequently
the disorder occurs in
a group of people
similar to you.
PPV =
83%
NPV =
99+%
PPV =
33%
NPV =
99+%
High-risk* group
The disorder occurs more frequently
(e.g., 1 in 100 or higher) in this group.
Out of 100 women with a
positive result, 83 will have
babies with Down syndrome,
and 17 will not have babies
with Down syndrome.
Women with a negative
result will only rarely
have a baby with Down
syndrome.
Out of 100 women with
a positive result, 33 will
have babies with Down
syndrome, and 67 will not
have babies with Down
syndrome.
Women with a negative
result will only rarely
have a baby with Down
syndrome.
Low-risk group
The disorder occurs less frequently
(e.g., 1 in 1,000 or lower) in this group.
*You are at “high risk” if you are 35 years or older; you have had an ultrasound exam that shows a possible problem with the fetus; you have had a previous child with one of these disorders;
you have a chromosomal problem that increases your risk of having a child with trisomy 21 or trisomy 13; or you have had a positive first-trimester or second-trimester screening test result.
Positive result Negative result Positive result Negative result
Understanding Your Results
CELL-FREE DNA PRENATAL
SCREENING TEST
BOTTOM LINE:
• Cell-free DNA testing is a very good screening test to detect common chromosomal disorders in high-risk women, but it has limitations.
• A negative result does not rule out the possibility of having a baby with a chromosomal disorder or other disorders that the cfDNA test
does not test for.
• If you have a positive result, a diagnostic test is needed to determine if the baby is truly affected. False-positive results are more
frequent among low-risk women than those at high risk.
• If you have cell-free DNA screening, a blood test or ultrasound exam should be offered to screen for neural tube defects and other conditions.
http://www.acog.org/Resources-And-Publications/Committee-Opinions/Committee-On-Genetics/Cell-Free-DNA-Screening-for-Fetal-Aneuploidy