Statistical analysis
PlGF concentration levels were converted to MoMs by performing a
regression analysis to allow for change with gestational age, weight,
smoking and method of conception (spontaneous or in vitrofertilization (IVF)) in the first 500 women screened. The PlGF MoM in
these 500 women were then retrospectively calculated and recorded in our
screening database. PlGF MoMs in subsequent screened pregnancies were
prospectively calculated along with the estimated risks for trisomy 21
as described below.
Trisomy 21 risks using PlGF alone or in conjunction with other screening
markers were determined using the multivariate Gaussian model approach
currently used as a standard method in risk calculation
software.11Reynolds TM, Penney MD. The mathematical basis of
multivariate risk screening: with special reference to screening for
Down’s syndrome associated pregnancy. Ann Clin Biochem 1990;27:452-8
The expected PlGF MoM distributions in unaffected and trisomy 21
affected pregnancies were based on an earlier collaborative study.
22Han J, Liu H, Xu ZP,
Cuckle H, Sahota D, et al. Maternal serum PlGF (placental growth
factor) in Chinese women in the first trimester undergoing screening
for Down syndrome. Eur J Obstet Gynecol Reprod Biol 2016;201:166-70
This study indicated that log10 PlGF MoM distribution
means and standard deviation in unaffected pregnancies were 0 and
0.1638, respectively, whilst corresponding figures in trisomy 21
affected pregnancies were 0.1979 and 0.1511.17 The
assumed correlation between log10 PlGF MoM and
log10 PAPP-A MoM, log10 free β-hCG and
log10 NT MoMs in unaffected pregnancies were 0.285,
−0.019 and −0.027, respectively, whilst that in trisomy 21 affected
pregnancies were respectively 0.0435, −0.121 and −0.337.17 Modeling in our earlier study indicated a screening
test based on maternal age, fetal NT, PAPP-A, free β-hCG and PlGF MoMs
was expected to have a 96.2% detection rate for trisomy 21 for a 5%
false positive rate. 17
Three risks for trisomy 21 were prospectively estimated, one based on
the conventional combined test, a second based on age, fetal NT, free
β-hCG and PlGF and lastly one based on age, fetal NT, free β-hCG, PAPP-A
and PlGF.
Area under the receiver operating characteristic (ROC) curve (AUC),
detection rates and false positive rates were calculated for the
combined risk as well as estimated risks based on replacement of PAPP-A
by PlGF as well as the addition of PlGF to the conventional combined
test. AUCs among the different first trimester combined tests were
compared using the Delong test. 33DeLong ER, DeLong DM,
Clarke-Pearson DL. Comparing the areas under two or more correlated
receiver operating characteristic curves: a nonparametric approach.
Biometrics. 1988;44: 837-45 McNemar test was used to determine if
replacement of PAPP-A by PlGF or the addition of PlGF resulted in a
significant number of pregnancies being reallocated with regard to risk
status for trisomy 21.
All trisomy 21 risks were estimated using our in house laboratory risk
calculation software whilst all other analyses were performed using SPSS
for Windows version 20 (SPSS, Illinois, USA) and MedCalc Statistical
Software version 18.10.2 (MedCalc Software bvba, Ostend, Belgium;
http://www.medcalc.org; 2018).