Published online 2011 Nov 29. doi: 10.3346/jkms.2011.26.12.1619
Mar 29, 2015 Non-invasive prenatal testing (NIPT) NIPT is an additional screening test that can tell you your risk of having a baby with Down's syndrome, Edwards' syndrome and Patau’s syndrome. Compared with other screening tests, such as the combined test, NIPT is more accurate and can be done earlier in your pregnancy (from about 9 to 10 weeks ).
PMID: 22148000
This article has been cited by other articles in PMC.
Abstract
The purpose of the current study was to propose a Korean-specific parameter set for calculating the risk of Down syndrome in the second trimester of pregnancy and to determine the screening performances of triple and quadruple tests in Korean women. Using the data on triple or quadruple screening from three hospitals in Korea during 7 yr, we re-converted the concentrations of four serum markers to multiple of median values according to gestational age and maternal weight. After re-calculating the risk of Down syndrome in each pregnancy by multiplying maternal age-specific risk by the likelihood ratio values for the serum markers, screening performances and optimal cut-off values of triple and quadruple tests were analyzed. Among 16,077 pregnancies, 23 cases had Down syndrome (1.4/1,000 deliveries). Compared to the previous program, the tests with new parameters had improved screening performance. The triple and quadruple tests had detection rates of 65.2% and 72.7%, respectively, at a false-positive rate of 5%. The optimal cut-off value for the quadruple and triple tests was 1:250. We have presented a Korean-specific parameter set for Down syndrome screening. The proposed screening test using this parameter set may improve the performance of Down syndrome screening for Korean women.
Keywords: Down Syndrome, Korean-Specific, Second Trimester Screening, Triple Test, Quadruple Test, Serum Marker
INTRODUCTION
Down syndrome is the most common chromosomal anomaly, with an incidence at birth of 1 per 800-1,000 (, ). Amniocentesis or chorionic villous sampling for prenatal chromosomal analysis is difficult to be performed in all patients because of the risk of fetal loss and the cost (). Accordingly, prenatal screening to identify pregnancies at increased risks for Down syndrome is very important.
Since Cuckle et al. () reported that a low level of serum α-fetoprotein (AFP) is a high-risk marker for Down syndrome in 1984, several maternal serum markers have been developed. Measuring maternal serum levels of AFP, total human chorionic gonadotrophin (hCG), and unconjugated estriol (uE3) is known as triple screening (, ). The quadruple test, which adds inhibin A, was introduced in the early 2000s (, ). To calculate the risk of Down syndrome using serum markers, commercially available software programs are used in practice. Because variances are observed between software programs, it is effective that each country has a software program to apply variances and covariances of serum markers for its own population to achieve accurate screening.
In Korea, the triple and quadruple tests have been used widely among pregnant women since December 2004 and October 2009 under the support of National Health Insurance, respectively. However, the accuracy of Down syndrome screening tests is questionable in Korea, because the software programs in use were mainly based on dataset compiled from Western women. In addition, little information is available on the performance of these screening tests for Korean women. Few reports have analyzed the performance of triple screening in Korea, but the sample sizes were small (9, 10).
Recently, ethnic differences in serum marker levels have been reported (-), and the maternal age-related risk for Down syndrome was also reported to be different among races (). Therefore, it is requested to establish screening tests specific for each race or region (). Accordingly, in this study, we determined the covariances of serum markers for triple and quadruple screening tests in a Korean population, and re-calculated the risk of Down syndrome using newly determined values. Then, we compared the performances with those by software currently in use. This study introduces a dataset for Down syndrome serum screening and cut-off values specific for pregnant Korean women.
MATERIALS AND METHODS
Study participants
We analyzed the medical records of all pregnant Korean women who underwent triple or quadruple screening test between 14 and 21 weeks gestation at Seoul St. Mary's Hospital and Yeouido St. Mary's Hospital (2002-2009), and Cheongwha Women's Medical Center (2005-2009). A total of 17,890 pregnant women had second trimester screening tests; 1,813 pregnant women who had no records on fetal outcomes were excluded and 16,077 pregnant women were analyzed. Based on the records of screening tests, we determined the serum levels of AFP, hCG, uE3, and inhibin A, and the expected risk of Down syndrome. For those pregnant women who underwent amniocenteses, we checked the karyotype results, and for the pregnant women who did not undergo amniocenteses, we investigated the presence of fetal Down syndrome using neonatal charts on the date of birth and 1 month after birth.
Screening performances of triple and quadruple tests based on the HIT program
Gestational age (GA) was estimated by the menstrual history if regular or by ultrasonographic scan. Maternal age referred to age at the time of expected delivery date. Maternal serum levels of AFP, hCG, uE3, and inhibin A were determined using the Unicel™ Dxl 800 Access Immunoassay System with reagents (Beckman Coulter®, Inc., Fullerton, CA, USA). The screening performances of the triple or quadruple tests based on the HIT program (Hamchoon Inc., Seoul, Korea) with a cut-off value of 1:270 were calculated.
Down syndrome risk assessment using newly established parameters of serum markers
To correct the variable changes in serum marker concentrations according to gestational age, the concentrations were converted to multiple of the median (MoM) values for the relevant gestational ages. To provide reliable medians, regression analysis of each serum marker on gestational age among unaffected pregnancies was performed using the median concentration of each serum marker and median gestation (in days) for pregnancies at each completed week of pregnancy, weighted for the number of women at each week. The MoMs were calculated separately in each hospital according to the median values obtained from following regression equations: AFP (ng/mL) = 10(0.7569 + 0.0078 * GA); hCG (IU/mL) = 10(6.5772 - 0.0733 * GA + 0.0003 * GA * GA); uE3 (ng/mL) = -4.4196 + 0.0499 * GA; Inhibin A (pg/mL) = 10(0.7569 + 0.0078 * GA).
And then, all of the MoMs for the four serum markers were corrected for maternal weight using the weighted median regression method. The median MoMs were calculated separately in the 14 maternal weight groups (at 5-kg intervals). The values were then weighted by frequency, and subjected to regression estimation together with the median maternal weight of the corresponding weight group. Among simple linear, quadratic, log-linear, and log-quadratic regression models, the most suitable model was based on the multiple determination coeffecients (the R square). The weight-adjusted MoMs were calculated according to the following formula: AFP corr = AFP MoM observed/AFP MoM expected, where MoM expected was calculated according to the selected regressed equation: AFP (MoMs) = 2.2856 - 0.0328 * kg + 0.0002 * kg * kg; hCG (MoMs) = 10(0.2636 - 0.0047 * kg); uE3 (MoMs) = 10(0.1848 - 0.0043 * kg + 0.00002 * kg * kg);. inhibin A (MoMs) = 10(0.3085-0.0078 * kg +0.00004 * kg * kg).
All weight-corrected MoM values were converted to log-equivalents to obtain the distribution parameters. Goodness-of-fit to log-Gaussian distribution for the marker values was judged by inspection of the log-probability plot for unaffected pregnancies and the Kolmogorov-Smirnov test for affected pregnancies. Upper and lower truncation limits were set within which the available data adequately fitted the Gaussian model judged by inspection of the log-probability plot. Values outside those limits were given MoM values at the appropriate limit. The truncation limits for markers were 0.5-2.0 MoM for AFP, 0.3-2.7 MoM for uE3, 0.5-1.7 MoM for hCG, and 0.5-2.1 MoM for inhibin A.
The mean and standard deviations for each marker in unaffected and affected pregnancies were calculated by using the log10 of the median as the mean. The risk of Down syndrome was assessed by a commonly used risk algorithm. The likelihood ratio (LR) obtained with each marker was the height of the Gaussian distribution for the Down syndrome pregnancies divided by the height of the Gaussian distribution for the unaffected pregnancies at the particular value of the variables concerned. Age-specific risk was derived from the previous report of maternal age-specific rates of Down syndrome in Korean pregnant women ().
The case-specific risks of Down syndrome in triple and quadruple tests were estimated using the following equations: risk with triple screening = risk age * LR (AFP) * LR (HCG) * LR (uE3); risk with quadruple screening = risk age × LR (AFP) * LR (HCG) * LR (uE3) * LR (inhibin A).
Statistical analysis
We compared the incidence of Down syndrome between pregnancies in which the maternal age was < 35 yr and pregnancies in which the maternal age was ≥ 35 yr by Student's t-test. The median concentrations and MoMs of the serum markers were compared with published values for Caucasian women for the relevant gestational age by calculating the ratio. The MoM values of serum markers between unaffected and Down syndrome pregnancies were compared using Student's t-test.
Detection and false-positive rates for Down syndrome were re-calculated for all pregnancies. In particular, to determine the optimal cut-off value, we constructed the area under the receiver operating characteristic (AUROC) curve. A P value < 0.05 was considered statistically significant. All statistical analyses were carried out using SPSS (version 12.0; SPSS Inc., Chicago, IL, USA).
Ethics statement
The study was approved by the institutional review board of the College of Medicine of the Catholic University of Korea (KC10RES10193, SC11RIM10074). The board waived informed consent from the subject patients. It was conducted in accordance with the Declaration of Helsinki.
RESULTS
Demographic characteristics
Among the 16,077 pregnancies, Down syndrome occurred in 23 cases (1.4/1,000 newborns). The demographic characteristics of the pregnancies included in this study are summarized in Table 1. The mean maternal age was 31.1 ± 3.5 yr, and the mean maternal weight was 57.2 ± 8.5 kg. Among the gravidas > 35 yr of age, the prevalence of Down syndrome was 5.0 per 1,000 newborns, which was significantly higher than the 0.7 per 1,000 newborns among the gravidas < 35 yr of age (P = 0.002).
Table 1
Demographic characteristics of all pregnancies and pregnancies complicated by Down syndrome
All data are expressed Number of pregnancies or mean ± SD. *Screening analysis using HIT program (Hamchoon Inc., Seoul, Korea) at a cutoff value of 1:270.
Triple and quadruple screening tests were performed on 8,805 and 7,992 pregnancies, respectively. Among the 23 fetuses with Down syndrome, 17 had positive triple or quadruple screening tests; 4 and 2 fetuses had negative triple and quadruple screening tests using the HIT program, respectively.
The serum concentrations and MoMs of four serum markers for Down syndrome and unaffected pregnancies
When the median values of the serum markers at each gestational age were compared with the published values for Caucasian women (), the median values of all the serum markers were higher in Korean women than Caucasian women. The range of ratios was highest for inhibin A (range, 1.5-1.9), followed by uE3, AFP, and hCG (Table 2).
Table 2
Comparison of median maternal serum concentrations of AFP, hCG, uE3 and inhibin A between Korean and Caucasian women with unaffected pregnancies
All data are expressed as median values. *Data for Caucasian women were reported by MacRae et al. (); †Ratio of the serum markers' medians calculated in this study to those in a published study with Caucasian women for the relevant gestational age. GA, gestational age; wk, week; AFP, α-fetoprotein; hCG, human chorionic gonadotrophin; uE3, unconjugated estriol.
The serum marker levels were converted to MoMs according to gestational age and maternal weight and then the MoM level of each serum marker was compared between gravidas with pregnancies complicated by Down syndrome and gravidas with unaffected pregnancies. The mean MoM levels of AFP and uE3 in pregnancies complicated by Down syndrome were significantly lower than in unaffected pregnancies (P = 0.012 and P = 0.001, respectively). In addition, the mean MoM levels of hCG and inhibin A in pregnancies complicated by Down syndrome were significantly higher than in unaffected pregnancies (P = 0.001 and P < 0.001, respectively; Table 3). The medians and standard deviations of the log-Gaussian distribution for each serum marker are summarized in Table 3. For Korean and Caucasian women with pregnancies complicated by Down syndrome (), the MoMs of serum markers were 0.82 and 0.74 for AFP, 1.80 and 2.05 for hCG, 0.74 and 0.70 for uE3, and 2.543 and 2.54 for inhibin A, respectively.
Table 3
Statistical variables of log transformed and untransformed Gaussian distributions of each serum marker, expressed in multiple of the median (MoM) values in Down syndrome and unaffected pregnancies
*Comparison between unaffected pregnancies and Down syndrome pregnancies; P = 0.012, 0.001, 0.001, and < 0.001, by Student t test for AFP, hCG, uE3, and inhibin A respectively; †The log10 means were estimated from the medians; ‡Values in parentheses were reported by Wald et al. (). AFP, α-fetoprotein; hCG, human chorionic gonadotrophin; uE3, unconjugated Estriol.
Screening performance for second trimester screening tests
Using the statistical distributions for each marker, we calculated screening performances for triple and quadruple screenings. Fig. 1 shows the 'ROC curves' that gives the detection and falsepositive rates for the triple and quadruple tests. The AUROC curve was highly significant (P < 0.001) for quadruple (AUROC, 0.966; 95% confidence interval [CI], 0.940-0.991) and triple tests (AUROC, 0.955; 95% CI, 0.927-0.983). Table 4 shows the observed screening performances of triple and quadruple tests according to various risk cut-off values. The quadruple test achieved a Down syndrome detection rate of 81.8%, and the odds of being affected given a positive result (OAPR) of 1:59 at a risk cut-off value of 1:250. The triple test achieved a Down syndrome detection rate of 69.5% at a risk cut-off value of 1:300, and the OAPR was 1:73.
Screening perfomances of triple and quadruple screening for Down syndrome risk. (A) ROC curve of quadruple screening (AUROC, 0.966; 95% confidence interval [CI], 0.940-0.991) and triple screening (AUROC, 0.955; 95% CI, 0.927-0.983) for Down syndrome risk. (B) Down syndrome detection and false-positive rates for quaduple and triple test.
Table 4
Screening performance for Down syndrome with second trimester screening tests according to various risk cut-off values
DR, detection rate (%); FPR, false-positive rate; OAPR, odds of being affected given a positive result.
When the screening performance using our dataset was compared with the screening performance using the HIT program, triple screening showed a slight decrease in the detection rate from 66.7% to 65.2%, but a larger decrease in the false-positive rate from 7.3% to 6.1%. Quadruple screening also lowered the false-positive rate from 7.8% to 6.6% while maintaining the detection rate at 81.8% (Table 5).
Table 5
Screening performances of triple and quadruple tests using dataset of this study compared with the HIT program
DISCUSSION
This study established a parameter set of serum markers for triple and quadruple screening tests in pregnant Korean women. This can improve the screening performance of pregnancies complicated by Down syndrome compared with the previous programs based on a parameter set for Western women. In the current study, the detection rates of triple and quadruple screening were 65.2% and 72.7%, respectively, at a false-positive rate of 5%. Compared with the current program, the screening performance using our dataset was much improved.
In Korea, the second trimester screening tests have used a cutoff value of 1:270 since the screening test was implemented. When the serum screening markers were under development, a cutoff value of 1:270 was used to maintain consistency with the existing cut-off value for previous screening using maternal age only, but nowadays when various screening programs are available, it is not appropriate to apply a cut-off value of 1:270 uniformly without considering the screening performance of each program. The risk cut-off of a screening test should be set specifically to each country in consideration of the performance of the screening test, the cost and safety of invasive diagnostic procedures, the prevalence of Down syndrome, and the age distribution of pregnant women in the country. The optimal cut-off value for the quadruple screening test using our parameter set is considered to be 1:250 (detection rate of 81.8% at a false-positive rate of 6.6%). In our analysis, a lower cut-off value of 1:350 led to 79 additional amniocenteses and resulted in the detection of 1 additional case of fetal Down syndrome. To improve the detection rate, adopting the first trimester combined screening may be efficient rather than lowering the cut-off value in the second trimester screeing test. Recent studies have suggested that a combination of first trimester screening and second trimester quadruple screening achieved a detection rate of 94%- 96% at a false-positive rate of 5% (, ).
In the current study, the concentrations of AFP, hCG, uE3, and inhibin A were higher on average than the concentrations established for the Caucasian population. Ethnic differences have been noted in comparisons of black, Caucasian, and Asian populations in Europe and the USA (, , ). It is known that Asian women have the highest levels of AFP, hCG, and uE3 (, ), and our study was in line with the previous studies. In addition, we emphasize that the serum level of inhibin A is also the highest in Asian women (9). With respect to inhibin A, even though some studies have reported that black women have higher levels of inhibin A than Caucasian women (), a comparison between Asian and Caucasian women has not been attempted thus far. Although the ethnic effect on screening for Down syndrome appears to be relatively minor because of the counterbalancing effect of multiple serum markers (), correction for ethnicity can have a significant impact on individual risk, which could alter clinical decision-making ().
The median MoM of hCG in Korean women with a Down syndrome pregnancy was 1.80, which was lower compared to 2.01-2.12 in Caucasian women (). In Chinese women, the median MoM of hCG in Down syndrome pregnancies was 1.4 (). Although these studies were limited by small sample sizes, the median MoM of each serum marker in Down syndrome pregnancies may reflect racial differences. Further research with larger samples of pregnancies complicated by Down syndrome is needed.
Meanwhile, factors affecting the performance of Down syndrome screening include the use of ultrasound scans to estimate gestational age, maternal weight, insulin-dependent diabetes, and smoking (-). Of these factors, insulin-dependent diabetes and smoking were not taken into account in this study, as the prevalence of insulin-dependent diabetics in Korea is extremely low, with a prevalence of 1.4 per 100,000, and very few Korean pregnant women smoke cigarettes.
In conclusion, we introduce a more accurate and efficient screening method for antenatal Down syndrome screening based on a Korean-specifc parameter set. With the proposed parameter model, quadruple screening can detect 81.8% of pregnancies complicated by Down syndrome in the second trimester with a false-positive rate of 6.6% at a cut-off value of 1:250 in Korea. Future research is needed to develop the specified guideline of genetic counseling based on the larger samples for Korean women.
AUTHOR SUMMARY
Korean-Specific Parameter Models for Calculating the Risk of Down Syndrome in the Second Trimester of Pregnancy
Ji Young Kwon, In Yang Park, Yong Gue Park, Young Lee, Guisera Lee and Jong Chul Shin
In Korea, the triple and quadruple screening tests for pregnancies complicated with Down syndrome have been used widely. However, little information is available on the performance of these screening tests for Korean women. In this large population study, we introduced a more accurate and efficient screening method for antenatal Down syndrome screening based on a Korean-specific parameter set.
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Articles from Journal of Korean Medical Science are provided here courtesy of Korean Academy of Medical Sciences
Published online 2018 Jan 1. doi: 10.1515/jomb-2017-0022
PMID: 30581343
Language: English | Serbian
Summary
Background
Genetic screening for chromosomopathy is performed in the first trimester of pregnancy by determining fetal nuchal translucency (NT), and the pregnancy associated plasma protein-A (PAPP-A) and free human chorionic gonadotropin (free-beta HCG) biomarkers in maternal serum.
Methods
We tested the sensitivity, specificity, positive and negative expected values of each marker with the aim of setting a model for prenatal screening readings. Statistical data treatment has been performed on a sample of 340 pregnant women with positive results of prenatal screening.
Results
Sensitivity of PAPP-A was 0.6250 (probability 62.50%), free beta HCG 0.5893 (58.93%), NT 0.1785 (17.85%). Specificity of PAPP-A was 0.5106 (probability 51.06%), free beta HCG 0.5246 (52.46%), NT 0.9718 (97.18%). Positive expected value of PAPP-A was 0.2011 (probability 20.11%), free beta HCG 0.1964 (19.64%), NT 0.556 (55.56%). Negative expected value of PAPP-A was 0.8735 (probability 87.35%), free beta HCG 0.8662 (86.62%), NT 0.8571 (85.71%). The NT marker has a significantly higher specificity, which means that its normal value will significantly reduce the final risk of trisomy 21. The sensitivity of NT is much lower than that of biochemical markers, which means that a pathological value of NT does not have a significant influence on the final risk, i.e. the significantly higher sensitivity of biochemical markers will reduce the final risk of trisomy 21.
Conclusions
The analyses stress the importance of using a software which has the possibility to separate the level of a biochemical risk by correlating PAPP-A and free beta HCG and, by adding the NT marker, calculate the level of a final risk of Down syndrome.
Keywords: prenatal screening, Down syndrome, sensitivity, specificity
Kratak sadržaj
Uvod
Prenatalni skrining na Daunov sindrom u prvom trimestru trudnoče radi se ultrazvućnim merenjem nuhalne translucencije fetusa (NT) i određivanjem fetoplacentalnih biomarkera u maternalnom serumu: pregnancy associated plasma protein-A (PAPP-A) i free human chorionic gonadotropin (free beta HCG).
Metode
Ispitana je senzitivnost, specifićnost, pozitivna i negativna predviđena vrednost svakog markera u cilju postavljanja modela tumaćenja prenatalnog skrininga i interpretacija patoloških vrednosti. Ispitivanje je rađeno na uzorku od 340 trudnica sa pozitivnim nalazom prenatalnog skri ninga gde je amniocentezom dobijen kariotip ploda.
Rezultati
Senzitivnost PAPP-A bila je 0,6250 (verovatnoča 62,50%), free beta HCG 0,5893 (58,93%), NT 0,1785 (17,85%). Specifićnost PAPP-A bila je 0,5106 (verovatnoča 51,06%), free beta HCG 0,5246 (52,46%), NT 0,9718 (97,18%). Pozitivna predviđena vrednost PAPP-A bila je 0,2011 (verovatnoča 20,11%), free beta HCG 0,1964 (19,64%), NT 0,556 (55,56%). Negativna predviđena vrednost PAPP-A bila je 0,8735 (verovatnoča 87,35%), free beta HCG 0,8662 (86,62%), NT 0,8571 (85,71%). Uticaj PAPPA i free beta HCG na konaćan rizik za trizomiju 21 je približno jednak. NT marker ima znaćajno veču specifićnost što znaći da če njegova normalna vrednost znaćajno oboriti konaćan rizik za trizomiju 21. Senzitivnost NT je mnogo manja od biohemijskih markera, što znaći da patološka vrednost NT neutiće znaćajno na konaćan rizik, odnosno znaćajno veča senzitivnost biohemijskih markera če oboriti konaćan rizik za trizomiju 21.
Zaključak
Ove analize ukazuju da je veoma važno koristiti softver za prenatalni skrining koji ima mogučnost da razdvoji posebno nivo biohemijskog rizika korelacijom PAPP-A i free beta HCG i dodavanjem NT markera izraćuna nivo konaćnog rizika za Daunov sindrom.
Ključne reči: prenatalni skrining, Daunov sindrom, senzitivnost, specifičnost
Introduction
Prenatal screening for Down’s syndrome is done in the first trimester of pregnancy between 11 and 14 weeks by the ultrasound measurement of nuchal translucency (NT-neck crease) and the determination of fetal maternal serum biomarkers: pregnancy-associated plasma protein-A (PAPP-A) and free beta human chorionic gonadotropin (free beta-hCG). The concentration of biochemical markers in maternal serum is converted to a multiple of the median (MoM) of unaffected pregnancies at the same gestation stage (, , , ). The measured serum concentrations of these placental products are affected by maternal characteristics, including maternal age, racial origin, weight, diabetic status, smoking and method of conception. The risk of Down’s syndrome is determined i.e. calculated by a combination of software processing of the maternal characteristics, biochemical and sonographic markers. As a cut-off risk indicating prenatal karyotyping, 1:270 is used which corresponds to a pregnant woman aged 35 (5).
There is no significant association between fetal NT and maternal serum free beta hCG and PAPP-A in either trisomy 21 or euploid pregnancies (). It has been estimated that the false-positive rate in genetic screening is about 5%, which has resulted in an increased number of invasive diagnostic procedures of prenatal karyotyping in risk free pregnant women with respect to age (). On the other hand, it is important to increase the sensitivity of prenatal screening to identify pregnancies suspicious for trisomy 21 at the age of non-risk in pregnant women as younger age may reduce the final risk (). The performance of different screening methods for trisomy 21 with a combination of maternal age, sonographic and biochemical markers has been tested. It was found that effective screening in the first trimester of pregnancy should have a detection rate of about 95% and a false-positive rate of less than 3% (). The aim of this study, however, is to define the interpretation of resulting risks and establish a model for the interpretation of pathological values of prenatal screening markers for trisomy 21. We tested the sensitivity, specificity, positive and negative expected values of each marker with the goal of setting a model for prenatal screening readings and interpretation of pathological values.
Methods
PAPP-A and free-beta HCG biomarkers have been read with IMMULITE 2000 SIEMENS which operates on the principle of chemiluminescence, using the original reagents. The processing of data and determination of the risk of trisomy 21 have been done with PRISCA 5 SOFTWARE. Statistical data treatment has been performed on a sample of 340 pregnant women with respect to age, all with positive results of prenatal screening, and the karyotyping of a fetus has been obtained with amniocentesis. Using a sensitivity analysis method, it has been determined with high probability that a pathological value of the marker implies the presence of risk. Using a specificity analysis method, it has been determined with high probability that a normal value of the marker implies the absence of significant risk. Using a positive expected value method, it has been determined with high probability that risk is present only if the marker implies so. Using a negative expected value method, it has been determined with high probability that significant risk is absent if the marker implies so.
Results
The study included a sample of 340 pregnant women with suspicious findings of genetic screening and finite risk of Down’s syndrome in the PRISCA software greater than 1: 250, which is indicated based on prenatal karyotyping. In a sample of 340 high-risk findings of the screening, there were 18 (6.1%) results with the pathological values of NT markers. Pathological PAPP-A values were found in 174 (59.1%) cases. Free beta-hCG showed extreme values in 168 (57.1%) pregnant women. Values of the markers have been reported in deviation from the median – MoM (multiple of median). The risk of Down’s syndrome is shown in PRISCA software at two levels, the risk of biochemical correlations of biochemical markers PAPP-A and free beta HCG and finite risk adding the ultrasound marker NT. The results show the effect of each marker in the formation of risk of Down’s syndrome, influence of biochemical markers on biochemical and finite risk and impact of NT marker on the final risk.
Table I
PAPP-A MOM normal value | PAPP-A MOM pathological value | |
---|---|---|
Risk-free | 98 | 86 |
Risk | 68 | 88 |
Sensitivity 0.5563 (probability 55.63%)
Specificity of PAPP-A 0.4815 (probability 48.15%)
Positive expected value of PAPP-A 0.4615 (probability 46.15%)
Negative expected value of PAPP-A 0.5759 (probability 57.59%)
Table II
Influence of free beta HCG marker on biochemical risk.
Normal value free beta HCG | Free beta HCG pathological value | |
---|---|---|
Risk-free | 109 | 77 |
Risk | 63 | 91 |
Sensitivity of free beta HCG 0.5909 (probability 59.09%)
Specificity of free beta HCG 0.5860 (probability 58.60%)
Positive expected value of free beta HCG 0.5416 (probability 54.16%)
Negative expected value of free beta HCG 0.6337 (probability 63.37%)
Table III
Influence of PAPP-A marker on the final risk (biochemical + NT).
PAPP-A MOM normal value | PAPP-A MOM pathological value | |
---|---|---|
Risk-free | 145 | 139 |
Risk | 21 | 35 |
Sensitivity of PAPP-A 0.6250 (probability 62.50%)
Specificity of PAPP-A 0.5106 (probability 51.06%)
Positive expected value of PAPP-A 0.2011 (probability 20.11%)
Negative expected value of PAPP-A 0.8735 (probability 87.35%)
Discussion
In the last two decades, there have been numerous reports about the detection rate for different methods of screening for trisomy 21. Detection rate of the risk of maternal age and fetal NT is 75–80%, while the risk for age and biochemical screening of PAPP-A and free beta HCG is 70%. The combination of age-related risk markers NT, PAPP-A and free beta HCG increases the detection of trisomy 21 to 85–95% (, ). The ability to achieve a reliable measurement of NT is dependent on the appropriate training of sonographers (). Biochemical analyzers provide auto mated, precise and reproducible measurements. Presenting selectively the biochemical and ultrasound screening in the first trimester and representing a separate risk of sonographic and biochemical markers give a much better insight than the pure presentation of their combination. This type of screening is achieved by a policy in which the first-stage screening is based on maternal age, fetal NT and either tricuspid or ductus venosus flow, and biochemical testing is then performed only in those with an intermediate risk. An alternative first trimester contingent screening policy consists of maternal serum biochemistry in all pregnancies followed by fetal NT only in those with an intermediate risk after biochemical testing ().
Table IV
Influence of free beta HCG marker on the final risk (biochemical + NT).
Free beta HGC normal value | Free beta HCG pathological value | |
---|---|---|
Risk-free | 149 | 135 |
Risk | 23 | 33 |
Sensitivity of free beta HCG 0.5893 (probability 58.93%)
Specificity of free beta HCG 0.5246 (probability 52.46%)
Positive expected value of free beta HCG 0.1964 (probability 19.64%)
Negative expected value of free beta HCG 0.8662 (probability 86.62%)
Table V
Influence of NT marker on the final risk (biochemical + NT).
NT normal value | NT pathological value | |
---|---|---|
Risk-free | 276 | 8 |
Risk | 46 | 10 |
Sensitivity of NT 0.1785 (probability 17.85%)
Specificity of NT 0.9718 (probability 97.18%)
Positive expected value of NT 0.5556 (probability 55.56%)
Negative expected value of NT 0.8571 (probability 85.71%)
In our study, we examined the significance of the difference between positive and negative values of risk on the one hand and, on the other hand, the normal and pathological values of markers when they apply to the analysis of contingency. It is expected that the results indicate the distinction between individual markers and combinations of risks that have emerged in the sample. According to the results of chi-square testing, there is a certain tendency for the connection between markers and risks so that the null hypothesis can be rejected at the probability level of 90%. However, specificity indicates that there are exceptions. The correlation between markers and risks is best tested through the analysis of sensitivity, specificity and the positive and negative predictive value.
The influence of PAPP-A and free beta HCG on the final risk of trisomy 21 is approximately the same. NT marker has a significantly higher specificity, which means that its normal value will significantly reduce the final risk of trisomy 21. The sensitivity of NT is much lower than that of biochemical markers, which means that a pathological value of NT does not have a significant influence on the final risk, i.e. the significantly higher sensitivity of biochemical markers will reduce the final risk of trisomy 21. The analyses stress the importance of using a prenatal screening software which has the possibility to separate the level of a biochemical risk by correlating PAPP-A and free beta HCG and, by adding the NT marker, the level of a final risk of Down’s syndrome. At these two levels a very different risk is often obtained, and the analytical methods of this study suggest a new model of reading the obtained risks.
Effective screening for Down’s syndrome can be achieved in the first trimester of pregnancy with a detection rate of about 95% and a false-positive rate of less than 3%.
Acknowledgment
For the purpose of this research, all data were generated using the Immulite 2000 analyser, PRISCA 5 SOFTWARE and Immulite 2000 PAPP-A and free beta HCG reagents, manufactured by Siemens Healthcare Diagnostics Inc.
Footnotes
Conflict of Interest Statement: The authors stated that they have no conflicts of interest regarding the publication of this article.
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