20.05.08 Maternal Serum Screening and Down Syndrome screening methods Flashcards
(39 cards)
What is the UK national Screening committee’s definition of screening
- The 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 from the disease or condition.
What is the NHS Fetal Anomaly Screening Programme (FASP)
-Offered to all pregnant women, to assess risk of Trisomy 21 and a number of fetal anomalies.
How was Downs syndrome risk originally based
- Advanced maternal age used to identify increased risk of Downs syndrome in pregnancy.
- However, detection rate was only 30%, with 5% false positive rate.
What method is now used to identify women with increased risk of Down syndrome
Maternal serum screening, ultrasound and maternal age
What should all women be offered regarding Downs syndrome screening
- Information to help them decide if they want screening or not
- A down syndrome screening test that meets national standards
- An ultrasound scan to check for physical abnormalities in fetus.
Describe the Down syndrome screening pathway
- Undertaken from 10 to 20 weeks of pregnancy
- Couples receive counselling about procedure, possible outcomes and decisions they may face.
- Screening is nuchal translucency measurement, maternal serum testing or both.
- Screen positive if greater to or equal 1:150
- If risk is above this then offered invasive testing (AF or CVS).
- 3 day TAT
What is prevalence
Number of individuals in a population with the target condition
What is sensitivity
A screens ability to positively identify individuals who have the target condition
What is specificity
A screens ability to not detect individuals who do not have the target condition.
What is the detection rate
Proportion of affected individuals with a positive screening result
What is a screen positive rate (SPR)
Proportion of individuals who will be given a high risk result following screening.
What is a false positive rate (FPR)
Proportion of unaffected individuals with a positive screening result. Complement to specificity. i.e Specificity= 100-FPR
What is a false negative rate (FNR)
Proportion of women who are given a lower risk result but have an affected pregnancy. Complement to sensitivity.
What is nuchal translucency
- Maximum thickness of the subcutaneous translucency between the fetal skin and soft tissue overlying the cervical spine.
- Measured between 11 and 14 weeks.
- Combined with maternal age and size of baby to calculate risk.
What NT is associated with aneuploidy
- NT ≥3.5mm
- Highest to lowest prevalence= 21, 18, 13, 45,X and triploidy.
- Also linked to structural malformations or genetic syndromes.
- Diagnostic test offered
What percentage of pregnancies have NT ≥3.5mm
1%
What is maternal serum screening
Measuring the levels of serum analytes that have passed from fetus to mother’s blood
-Multiples of Median (MoM) are calculated for each serum marker
-
How is risk determined using maternal serum screening
- Multiples of Median (MoM) (serum marker concentration for a pregnant woman) is divided by the median concentration value for unaffected pregnancies of the same gestational age.
- Mother’s serum results, age, weight and gestation all used in risk calculation
Why is the median marker level used (rather than mean) for serum results
-Median is not skewed by very high or very low values.
What screening strategy is recommended
- Combined screening
- serum screening (β-hCG, PAPP-A) and NT
- Can be done at 10-14 weeks
- If screen positive result received, a woman should have rapid access to appropriate counselling.
- Looks for trisomy 13, 18 and 21
What factors affect serum screening
- Serum marker levels tend to be decreased in heavier women and increased in lighter women.
- Serum marker levels tend to be higher in Afro-Caribbean women than Caucasian.
- IVF can alter levels (β-hCG and hCG levels higher in IVF pregnancies).
- AFP and uE3 levels tend to be lower in women with insulin dependent diabetes mellitus
- Smokers have lower PAPP-A, free β-hCG and hCG levels
What kind of screening is recommended for twin pregnancies
- Quadruple screening
- AFP, β-hCG, uE3+ Inhibin A
- Done at 14-20 weeks
What risks are there for screening twin pregnancies
- Marker levels raised in twin pregnancies
- One twin could be affected and the other not
- Dichorionic twins and monochorionic twins have different serum levels
- Procedural risk for prenatal testing is higher for twins.
What else is the quadruple test used for
- Downs syndrome testing
- When NT is not available, not as accurate as combined.
