Screening Markers and Tests
αlpha can be used with up to 12 different markers for first trimester (10 weeks to 13 weeks 6 days) and second trimester (14 weeks to 22 weeks 6 days) screening. It is supplied ready to use for following markers for first trimester, second trimester, Integrated and sequential screening:
| Marker | Down's Syndrome | Neural Tube Defects | Trisomy 18 | Trisomy 13 | SLOS | Pre-eclampsia |
| First trimester | ||||||
| Plasma Protein A (PAPP-A) | ✔ | ✔ | ✔ | ✔ | ||
| Total human Chorionic Gonadothrophin (T-hCG) | ✔ | ✔ | ✔ | |||
| Free β-hCG | ✔ | ✔ | ✔ | |||
| Placental Growth Factor (PlGF) | ✔ | ✔ | ||||
| Nuchal Translucency (NT) | ✔ | ✔ | ✔ | |||
| Ductus Venosus Pulsatility Index (DVPI) | ✔ | |||||
| A nasal bone examination (NBE) | ✔ | |||||
| Ductus Venosus blood flow (DVBF) | ✔ | |||||
| Mean arterial pressure (MAP) | ✔ | |||||
| Uterine artery pulsatility index (UAPI) | ✔ | |||||
| Second trimester | ||||||
| Alphafetoprotein (AFP) | ✔ | ✔ | ✔ | ✔ | ✔ | |
| Unconjucated Estriol (uE3) | ✔ | ✔ | ✔ | ✔ | ||
| T-hCG | ✔ | ✔ | ✔ | ✔ | ||
| Free β-hCG | ✔ | ✔ | ✔ | |||
| Inhibin-A | ✔ | ✔ | ✔ | |||
| Placental Growth Factor (PlGF) | ✔ |
The following provides information about the expected screening performance for each condition:
Screening tests for Down's syndrome
αlpha can be used to screen for Down's syndrome using the screening markers in any combination including those in the table below (Reference 1):
| Test | Markers |
|---|---|
| Integrated | Maternal age with nuchal translucency and PAPP-A in the first trimester, and AFP, uE3, total or free ß hCG and inhibin-A in the second trimester. Measurement of first trimester PlGF, DVPI and a nasal bone examination can optionally be included. Click for more information. |
| Serum integrated | Maternal age with PAPP-A in the first trimester, and AFP, uE3, total or free ß hCG and inhibin-A in the second trimester. Measurement of first trimester PlGF can optionally be included. Click for more information. |
| Combined | Maternal age with first trimester nuchal translucency, total or free ß hCG and PAPP-A. Measurement of first trimester PlGF, DVPI and a nasal bone examination can optionally be included. Click for more information. |
| Quadruple | AFP, uE3, total or free ß hCG and inhibin-A |
Ductus venosus blood flow can also be included as a categorical variable in the screening interpretation (Reference 5).
Screening performance
At a False Positive Rate of 5%, the estimated Detection Rate for the Integrated test, serum integrated test, combined test and quadruple test are 96.5%, 90%, 90.4% and 86% respectively (references 1,11,12 & 13). First trimester markers were measured at 11 weeks gestation and the 2006-8 England and Wales age distribution was used.
The figure below show a plot of the false-positive rate against detection rate for the four tests (Reference 1)
Measurement of first trimester PlGF, DVPI and a nasal bone examination improves the screening performance of first trimester and Integrated tests. Click for more information
Single risk
Alpha can optionally screen using a single risk of Down's syndrome, trisomy 18 or trisomy 13 or using Down's syndrome risk and a single risk of trisomy 18 or trisomy 13 (reference 9)
Open neural tube defect screening
AFP can be used for screening for open neural tube defects between 15 weeks and 22 weeks 6 days. The estimated detection rate for open spina bifida at gestational ages between 15 and 20 weeks is given in the table below (from reference 8)
| Detection Rate (%) Gestational age (weeks) |
||||||||
|---|---|---|---|---|---|---|---|---|
| Method used to estimate gestational age |
AFP (MoM |
15 | 16 | 17 | 18 | 19 | 20 | FPR (%) 15-20 weeks |
| Dates | ≥2.0 | 68 | 77 | 81 | 82 | 78 | 70 | 3.4 |
| ≥2.5 | 57 | 68 | 73 | 73 | 69 | 59 | 0.8 | |
| ≥3.0 | 47 | 59 | 64 | 65 | 60 | 50 | 0.2 | |
| BPD | ≥2.0 | 83 | 89 | 92 | 92 | 90 | 84 | 2.0 |
| ≥2.5 | 74 | 83 | 86 | 87 | 84 | 76 | 0.3 | |
| ≥3.0 | 66 | 76 | 80 | 81 | 77 | 68 | 0.1 | |
Screening tests for trisomy 18
αlpha can be used to screen for trisomy 18 using maternal age with second trimester measurements of AFP, uE3, total hCG and Free β-hCG, maternal age with first trimester measurements of PAPP-A, total hCG, Free β-hCG and NT and maternal age with nuchal translucency and PAPP-A in the first trimester, and AFP, uE3, total or free ß hCG in the second trimester combined in an Integrated test. As an alternative to screening the risk of a pregnancy being affected with trisomy 18 can be shown on the report as an incidental result of screening for Down's syndrome.
The estimated trisomy 18 detection rate and false positive rate at an early second trimester cut-off of 1 in 100 is given in the table below. The NT measurement was taken at 11 weeks and the maternal age distribution is that for England & Wales in 2006-2008. (References 4, 10 & 13).
| Test | Detection rate (%) | False positive rate (%) |
|---|---|---|
| Combined (NT, Free β-hCG & PAPP-A) |
92 | 0.19 |
| Triple (AFP, uE3 and Free β-hCG) |
48 | 0.11 |
| Serum integrated (PAPP-A in the first trimester and AFP, uE3 and Free β-hCG in the second trimester) |
84 | 0.12 |
| Integrated (NT and PAPP-A in the first trimester and AFP, uE3 and Free β-hCG in the second trimester) |
87 | 0.04 |
Screening tests for trisomy 13
αlpha can be used to screen for trisomy 13 using maternal age with first trimester measurements of PAPP-A, Free β-hCG and NT or maternal age with a second trimester measurement of inhibin-A and first trimester measurements of PAPP-A and NT combined in a Integrated test . As an alternative to screening the risk of a pregnancy being affected with trisomy 13 can be shown on the report as an incidental result of screening for Down's syndrome.
The estimated trisomy 13 detection rate and false positive rate at an early second trimester cut-off of 1 in 100 is given in the table below. The NT measurement was taken at 11 weeks and the maternal age distribution is that for England & Wales in 2006-2008. (References 4 & 13)
| Test | Detection rate (%) | False positive rate (%) |
|---|---|---|
| Combined (NT, Free β-hCG & PAPP-A) |
67 | 0.21 |
| Serum integrated (PAPP-A in the first trimester and inhibin-A in the second trimester) |
44 | 0.22 |
| Integrated (NT and PAPP-A in the first trimester and inhibin-A in the second trimester) |
68 | 0.10 |
Screening tests for pre-eclampsia
The pre-eclampsia prevalence together with:
- second trimester measurements of AFP, uE3, total hCG, Free β-hCG, inhibin-A and PlGF
- first trimester measurements of PlGF, PAPP-A, mean arterial pressure (MAP) and uterine artery pulsatility index (UAPI)
- previous history of pre-eclampsia, family history of pre-eclampsia and parity
can be used to calculate the risk of developing all pre-eclampsia or early pre-eclampsia (pre-eclampsia occurring before 34 weeks gestation). This provides an opportunity for closer monitoring of the pregnancy and preventive treatment in patients with a risk above a specified cut-off.
In the following screening performance estimates the prevalence of all and early pre-eclampsia is taken to be 4% and 0.3% respectively and the detection rate is given for a false positive rate of 5%:
Using the quadruple test markers plus a measurement of second trimester PlGF the estimated detection rate is 45% (all pre-eclampsia) and 49% (early pre-eclampsia). If previous history of pre-eclampsia, family history and parity are taken into account, the estimated detection rate is 48% (all pre-eclampsia) and 51% (early pre-eclampsia). (Reference 2)
Using first trimester measurements of PAPP-A, PlGF and MAP the estimated detection rate is 28% (all pre-eclampsia) and 55% (early pre-eclampsia). If previous history of pre-eclampsia, family history and parity are taken into account, the estimated detection rate is 35% (all pre-eclampsia) and 58% (early pre-eclampsia). If a measurement of uterine artery pulsatility index (UAPI) is also taken, and previous history of pre-eclampsia, family history and parity are taken into account, the estimate detection rate is 40% (all pre-eclampsia) and 71% (early pre-eclampsia).
Using first trimester measurements of PAPP-A and MAP together with the quadruple test markers plus a measurement of second trimester PlGF the estimated detection rate is 51% (all pre-eclampsisa) and 66% (early pre-eclampsia). Taking the PlGf measurement in the first trimester reduces the detection rate by 6% for all pre-eclampsia. If previous history of pre-eclampsia, family history and parity are taken into account, the estimated detection rate is 56% (all pre-eclampsia) and 68% (early pre-eclampsia) (derived from references 2,6 and 7)
Smith Lemli Opitz syndrome (SLOS)
Pregnancies at high risk of SLOS can be identified using second trimester measurements of AFP, uE3 and total hCG as an incidental result of Down's syndrome screening. (Reference 3)
References
1. Improvements in antenatal screening for Down’s syndrome (2013)
2. Screening for pre-eclampsia using serum placental growth factor and endoglin measurement with Down’s Syndrome Quadruple test markers (2012)
3. Assigning risk for Smith-Lemli-Opitz syndrome as part of 2nd trimester screening for Down's syndrome (2002)
4. Screening for trisomy 18 and trisomy 13 using first and second trimester Down syndrome screening markers (2013)
5. Adding ductus venosus blood flow as a categorical variable to the Combined and Integrated tests in Down’s syndrome screening (2012)
6. Prediction of early intermediate and late pre-eclampsia from maternal factors, biophysical and biochemical markers at 11-13 weeks (2011)
7. Competing risks model in early screening for preeclampsia by biophysical and biochemical markers (2013)
8. Neural tube defects (2000)
9. Antenatal screening for Down’s syndrome, trisomy 18, and trisomy 13: Reporting a single screening result for all three (2015)
10. Serum marker truncation limits in first trimester antenatal screening for trisomy 18. (2018)
11. Prenatal screening for Down syndrome in twin pregnancies: Estimates of screening performance based on 61 affected and 7302 unaffected twin pregnancies (2018)
12. Corrigendum for Wald NJ, Bestwick JP, Huttly WJ, Aldis J, Borrell A, Goodburn S & Mills I (2018) https://doi.org/10.1002/pd.5381 (2019)
13. Antenatal screening for Down’s Syndrome: Revised nuchal translucency upper truncation limit due to improved precision of measurement (2020)