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Nuchal Translucency Screening
Nuchal translucency is the measurement of the space from the back of the fetal
neck to the skin overlying the neck. This refers to an observation that
abnormal fetuses tend to show an accumulation of fluid in this region and
have an increased risk of having a variety of chromosome abnormalities present,
including the common finding of Down syndrome, or Trisomy 21.
After this finding was first reported, an
extended study was initiated by Kypros Nicolaides
in London, England which demonstrated that through
using an increased NT measurement, 78% of Down
syndrome pregnancies could be detected with
a 5% false positive rate. This research institute
grew into the Fetal Medicine Foundation which
now provides training and certification for
this ultrasound technique.
Biochemical Testing
While many biochemical proteins can be used in prenatal testing, the goal in
selecting biochemical markers is to obtain the best balance between detection
efficiency and false positive rate. This is accomplished by picking markers
with the greatest difference between levels found in affected and unaffected
pregnancies. These two proteins are widely accepted to be PAPP-A (pregnancy
associated plasma protein) and the free-lying beta subunit of hCG (freeBeta™). Human
chorionic gonadotropin (hCG) is comprised of an alpha and beta subunit, with
the beta subunit produced in significant excess in Down syndrome pregnancies. PAPP-A
(Pregnancy Associated Plasma Protein) is another marker with a significant
difference in levels seen between a normal pregnancy and a Down syndrome
affected pregnancy. Screening for these two biochemical analytes alone (without
the NT measurement) allows for a 65% detection of Down syndrome in the first
trimester.
When these two screening modalities are combined
in an Ultrascreen, between 85-91% of Down syndrome
pregnancies can be detected between 11-14 weeks
of pregnancy as well as 40% detection of cardiac
abnormalities and >97% detection of other
chromosome abnormalities. This compares very
favorable to the 65-70% detection rate seen
in the current method of serum screening using
the triple screen of hCG, AFP and estriol.
Calculating a Risk-Why is Ultrascreen™ the
Best Test?
All Down syndrome screening protocols designed for women less than age 35 years
of age begin with the prior maternal age related risk. Biochemical or ultrasound
markers are then used to adjust this original risk (a priori risk) to determine
a final risk. The screening risk cut-off (the risk at which a diagnostic
procedure should be offered) is generally the risk of Down syndrome equivalent
to that of a 35 year old (1/380 risk at term).
Traditional triple screen testing typically
yields a 65% detection rate for Down syndrome
and has an 8% false positive rate. Conversely,
Ultrascreen provides a 90% detection rate
with a false positive rate of 5%. This allows
the highest detection while indicating the
fewest women for invasive testing of any screening
program.
The statistical analysis of the results of
this testing is further enhanced by using a
sophisticated means of calculation known as
Bayesian analysis, which permits a greater
power to discern normal from abnormal results. The
result of this method of calculation is that
Ultrascreen provides not only the highest detection
of Down syndrome, it also provides the lowest
false positive
No other form of first trimester testing can
provide greater yields in detection rates and
false positive rates as compared with Ultrascreen.
Dried Blood Spot Card Sample
Collections
Another unique feature of this test is the manner of blood collection. Blood
is collected onto filter paper using a simple fingerstick. This blood can
then be extracted back out in the laboratory for testing, and is a better form
of blood collection for several reasons. The blood proteins are stabilized
in a dried format which protects against degradation of the specimen. This
provides better data, and in turn, a better result.
Not only does the laboratory benefit, but
so does the doctor’s office and the patients
as well. Samples can be collected by anyone,
as a fingerstick test is a CLIA waved test. This
means no special training of personnel. It
means no phlebotomy for the difficult patient
to draw, and it is easier for specimen transportation.
The advantages of dried blood spot technology
include:
- Simplification of the blood collection
process
- no syringe needles required
- Simple, low cost shipping
- Reduced specimen degradation
- Greater screening efficiency
- reduced variation in protein levels due
to greater protein stability
Why screen in the First Trimester?
The main advantage of a first trimester screening program is peace of mind. This
is a time when a pregnant woman can find out if her pregnancy is likely to
be normal or not, and can evaluate further testing options. This is of great
value to most women who seek the reassurance this testing can provide.
How can I implement this new test into my
practice?
There are several ways to allow your patients to access these services. You
may choose to refer your patients to a number of referral practices which accept
patients wishing these services. You may also choose to begin offering these
services yourself into your practice. This requires the ultrasonographer to
receive the initial training, whether by attending a course in person, or via
an online approach. In addition, images must be submitted, demonstrating proficiency
in this new technique. Finally, a laboratory provider of the PAPP-A/freeBeta
biochemistry will arrange for you to have the blood samples collected. Contact
us so we can assist you in deciding what is best for you and your patients.
Becoming a Provider of Ultra-Screen™ First
Trimester
Nuchal Translucency Film Submission Form First Trimester Nuchal Scan Patient Consent
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