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          
Ductus Venosus blood flow          
Mean arterial pressure          
Second trimester            
Alphafetoprotein (AFP)  
Unconjucated Estriol (uE3)    
T-hCG    
Free β-hCG      
Inhibin-A      
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%, 90%, 88% and 86% respectively.

The figure below show a plot of the false-positive rate against detection rate for the four tests (first trimester markers measured at 11 weeks) (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.  (Reference 4).

 
Test Detection rate (%) False positive rate (%)
Combined
(NT, Free β-hCG & PAPP-A)
79 0.10
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)
78 0.08
Integrated
(NT and PAPP-A in the first trimester and
AFP, uE3 and Free β-hCG in the second trimester)
82 0.05

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.  (Reference 4)

 
Test Detection rate (%) False positive rate (%)
Combined
(NT, Free β-hCG & PAPP-A)
65 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)
65 0.17

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 and mean arterial pressure (MAP)
  • 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 occuring 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).

 

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. (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)