VALUE OF THE COMBINED TEST
IN PRENATAL DIAGNOSTICS
Lončar D
*Corresponding Author: Dragan Lončar, Gynecology and Obstetrics Clinic, Clinical Center
Kragujevac , Vojislava Kalanovića 1A/3, 34000 Kragujevac, Serbia; Tel.: +381-64-616-8999;
E-mail: drloncar@sezampro.rs
page: 53
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DISCUSSION
In our study, we had nine (2.84%) pregnant
women with numeric aberrations in total and seven
(2.21%) pregnant women with structural aberrations
in fetuses which could be explained by the fact that
the sample was preselected. All pregnant women
were sent to Genetic Counseling at GOC, CC
Kragujevac (Table 2), for some suspicious reason
(positive personal and/or family case history, age
of the pregnant woman, giving birth to child with
chromosomal aberrations and/or fetal anomalies
in previous pregnancies, etc.). Similar results were
reported in the study conducted in Great Britain
in 2000, stating that the total incidence of Down’s
syndrome 2, 1 in 1,000 deliveries, which was 50.0%
more than in the national reports [4].
The importance of the NT measurement in
screening for Down’s syndrome during the first
trimester of the pregnancy was recognized back in
1990. With the limit value of 3 mm NT thickness,
the detection rate (DR) is 64.0% [5,6]. Screening
sensitivity of chromosomopathies in comparison to
NT was 75.0% with the value of false-positive ratio
of 2.1% [6]. In our sample, 11 pregnant women in
total from the group of 16 had a measured value of
NT above the median for the given CRL in the group
of pathological karyotypes that was 68, 75.0%. By
the analysis of the total sample, we found that with
26 pregnant women we measured a NT of 2.55 mm
above median for the given CRL and by invasive diagnostics we confirm 16 cases of chromosomal
fetal aberrations or 61.54% (Table 3). Methodology
of the combined test (Table 1) indicated that the
ultrasound screening was done first, and after that
to set the level of free β-HCG and PAPP-A, where
risks are calculated as the combination of the two
data [7]. For a certain gestation time, the levels of
free β-HCG and PAPP-A represent the factor of
probability, which is multiplied by the initial risk
in order to calculate the new risk [8]. Differences
in the concentration of free β-HCG between normal
pregnancies and those with trisomy 21 is increasing,
and differences in the level of PAPP-A is decreasing
with the length of the pregnancy.
There is no significant connection between
thicknesses of the fetal NT, level of free β-HCG
or PAPP-A in maternal serum in pregnancies with
trisomy 21 in relation to normal pregnancies, so
ultrasound and biochemical markers can be combined
in order to get more efficient screening results.
Numerous studies have confirmed the connection
between the low level of PAPP-A and trisomy 21
during the first trimester [2]. In normal pregnancies,
the level of PAPP-A in maternal blood increases with
gestation time, and in pregnancies with trisomy 21
it decreases [multiple of median (MoM)] (<0.5). By
setting the value of PAPP-A, it is possible to detect
52.0% of Down’s syndrome cases with 5.0% falsepositive
results [2]. In pregnancies with trisomy 21,
the level of free β-HCG is increased between 8 and
14 weeks of gestation. The level of free β-HCG in
maternal blood decreases normally with gestation
time, and in pregnancies with trisomy 21, the level
of free β-HCG increases (MoM >2.0) [9,10]. On the
basis of free β-HCG level, DR amounts to 42.0%
with 5.0% false-positive findings [11]. Frequency
of false-positive results, according to the available
literature, is estimated at 5.0% [12-15].
Our research has shown that the rate of the
false-positive findings is 1.0%, and that free β-HCG
is a more sensitive predictor than PAPP-A. Other
investigators, have reported identical conclusions
[14]. Predictive value of the individual biochemical
markers is represented at charts 1 and 2 by setting
the area below the ROC curve. On reviewing the
combined test predictive value of our sample of
pregnant women, we found the following results:
sensitivity of the test is 94.0%, specificity is 99.0%. The positive predictive value of the test is 0.83, and
negative predictive value of the test is 0.99 (99.0%).
The positive likelihood ratio (LR+) is 94.00 and
negative likelihood ratio (LR–) is 0.06 (Tables 4
and 5). We have already confirmed the published
positive qualifications of this screening method
[11,16] and point to its justification in every day
clinical practice [17] regarding posttest odds rate in
case of positive screening increases several times
over (almost 90 times). In the available literature, we
have not found any reports that have the calculation
of the credibility of the combined test and prediction
of posttest odds of this screening method. It is most
important to tell the patient(s) that this is a process
of screening and not the final diagnosis. That can be
given only on the basis of invasive intervention and
defining of the fetal karyotype [18].
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