18.05.08 Maternal serum screening and Down syndrome screening Flashcards
Give a definition of screening
UKNSC: is a process of identifying apparently healthy people who may be at increased risk of a disease or condition.
They can then be offered information, further tests and appropriate treatment to reduce their risk and or any complications arising from the disease or condition.
What is the aim of the NHS foetal anomaly screening programme?
The NHS Fetal Anomaly Screening Programme offers screening to all pregnant women in England to assess the risk of trisomy 21 and a number of fetal anomalies.
FASP aims to ensure that there is equal access to uniform and quality-assured screening, and that women are provided with high quality information so they can make an informed choice about their screening options and pregnancy. FASP is overseen by the UK NSC and is part of Public Health England.
What is the associated between advanced maternal age and Down syndrome risk?
Risk of Down syndrome pregnancies advances with age. Use as the sole criteria for risk gives a detection rate of 30% for a 5% false positive rate.
When can screening for Down syndrome be undertaken?
From 10-20/40
Define prevalance
Prevalence – The number of individuals in a population with the target condition
Define sensitivity and specificity
Sensitivity – A screen’s ability to refer individuals (for further tests) who do have the target condition. (proportion of +ves which are correctly ID’d as such)
Specificity – A screen’s ability to NOT to refer individuals who do NOT have the target condition (Proportion of –ves which are correctly ID’d as such)
Define detection rate and screen positive rate
Detection Rate (DR) – The proportion of affected individuals with a positive screening result.
Screen Positive Rate (SPR) - The proportion of individuals who will be given a high risk result following screening
Define false positive rate and false negative rate
False Positive Rate (FPR) - The proportion of unaffected individuals with a positive screening result. It is the complement of specificity ie: Specificity=100-FPR
False negative Rate (FNR) – The proportion of women who are given a lower risk result but have an affected pregnancy. It is the complement to the sensitivity.
What is the nuchal translucency? When is it measured?
Measured between 11+2 and 14+1 weeks, sonographer takes an US measurement (in mm) of the NT
NT is the maximum thickness of the subcutaneous translucency between the fetal skin and the soft tissue overlying the cervical spine (fluid under the skin at back of the fetal neck).
What size of NT is associated with aneuploidy?
A NT of ≥3.5mm is strongly associated with aneuploidy (+21,18,13, 45,X and triploidy in order of highest to lowest prevalence). NT of ≥3.5mm with a normal karyotype can be associated with a range of structural malformations (often Cardiac), and or genetic syndromes.
If the NT is >3.5mm between 11 and 14 weeks (found in ~1% of pregnancies) the pregnancy will be considered at an increased risk and a diagnostic test will be offered. Increased NT reflects foetal heart
failure and is strongly associated with a chromosomal abnormality.
What are multiples of the mean?
Measures levels of serum analytes (markers) that have passed from fetus into mother’s blood
Multiples of the Median (MoM) are calculated for each serum marker
MoM – the serum marker concentration for a pregnant woman, divided by the median concentration value for unaffected pregnancies of the same gestational age
Why is the MoM used instead of mean?
The median is used instead of the mean because it is not skewed by very high or very low values
Which markers are used in the combined screen? When is this performed and what is the detection rate?
Combined Screening - β-hCG, PAPP-A+ NT - The recommended test
Serum screening is performed between 10+0 and 14+1 (ideally at 11+2).
DR ~87% for 5% FPR
DR 90% for SPR of 2.2%
Which markers are used in the quadruple screen? When is this performed and what is the detection rate
Quadruple Screening – AFP, β-hCG, uE3+ Inhibin A
Performed at 14+2 to 20 weeks gestation (second trimester)
The recommended test for women who present too late for combined screening and in some trusts where combined screening has not yet been introduced
DR ~81% for 5% FPR
DR 75% for 3% SPR
Other than the Quad and combined screening tests, what other serum screening strategies exist?
1) Integrated Test - PAPP-A, β -hCG and NT measurement in 1st trimester followed by AFP, uE3 and inhibin A in second trimester (DR ~96% for 5% FPR)
2) Serum Integrated Test - PAPP-A and β -hCG at 11+0-13+6 wks followed by AFP, uE3 and inhibin A at 5-20 wks (Integrated test without NT) (DR ~88% for 5% FPR)
3) Contigency test
4. Stepwise sequential
