Prenatal Screening & Diagnosis Flashcards
What is the purpose of screening tests?
- screening tests are used to look for an abnormality / certain condition
- they are NOT definitive and another test is required to confirm the diagnosis
- they are used to dictate the clinical pathway and determine whether further testing is required
What do screening tests rely on?
- they rely on factors produced by the foetus / placenta that diffuse across the placenta and enter the maternal circulation
- increased or decreased levels of these factors may indicate an abnormality
- increased / decreased levels of the same factor can be indicative of different conditions, so the change from normal is measured
What is meant by diagnostic testing?
- diagnostic tests are used to confirm a suspicion of an abnormality / condition
- they have a high degree of specificity but there are also associated risks
What are the 2 main reasons why screening tests are important?
- to determine the health / condition of the unborn foetus
- to avoid / manage potentially untoward outcomes for the foetus, mother or both
- it allows for parents to make informed choices about their pregnancy, depending on whether it is deemed to be high-risk or not
What is the NICE recommendation for prenatal screening in England?
- all pregnant women are offered Down’s syndrome screening in the first trimester
- this can be performed via 2 different methods, depending on when the woman presents:
- the combined test
- the quadruple serum test
What is involved in the combined test for Down’s syndrome?
What is the detection rate?
- this involves a nuchal translucency measurement and maternal serum markers at 12 weeks
- the serum markers measured are:
- human chorionic gonadotrophin (hCG)
- pregnancy-associated plasma protein A (PAPP-A)
- it has an 80% detection rate and gives an early indication, but is also associated with 5% false positive rate
What is involved in the quadruple serum test and when can this be performed?
- this involves a blood test to measure 4 serum factors:
- human chorionic gonadotropin (hCG)
- alpha-fetoprotein (AFP)
- inhibin-A
- estriol (E3)
- it can be performed up to 24 weeks, but is most accurate between 15-18 weeks
When is the quadruple serum test used?
What is its drawback over the combined test?
- the quadruple test is less accurate than the combined test
- it can be used for women who present later
- if the results of the combined test are positive, the woman will still be sent for a quadruple serum test at 15-18 weeks
What are both the quadruple serum and combined test followed up by?
- both tests are followed up by a foetal anomaly USS at approx 20 weeks (18-21)
During which period can the combined test be performed?
What 2 conditions can the results raise a suspicion of?
- usually performed at 12 weeks, but can be performed between 10-14 weeks
- can be used to raise suspicion of Down’s syndrome (trisomy 21) and Edward’s syndrome (trisomy 18)
- PAPP-A is decreased from the norm in both trisomy 21 and 18
- hCG is elevated in trisomy 21
- hCG is depressed in trisomy 18
What factors are measured in the quadruple serum test and what conditions does this screen for?
- performed in 2nd trimester between 15-22 weeks to look for levels of:
- hCG
- estriol (E3)
- alpha-fetoprotein (AFP)
- + / - Inhibin A
- elevated levels of AFP indicate neural tube defects (NTDs)
- AFP, E3 and hCG are all depressed in trisomy 18
- AFP and E3 are depressed in trisomy 21, but hCG and inhibin A are elevated
Why is accurate dating of pregnancy vital for interpreting screening tests?
- accurate dating of pregnancy is vital as levels of serum markers are dependent on foetal age
- there are peaks and troughs of serum markers throughout pregnancy, so values need to be compared to what is expected at that point in pregnancy
What are the 3 major benefits of performing USS?
- safe as it is non-ionising
- cheap to perform
- image quality and detail have improved massively recently
- X-ray and CT scans may be more helpful in diagnosis, but they are radiating and carry a risk of harm to the unborn foetus
How is gestational age estimated using USS?
Why is this clinically important?
- the most accurate way to date a pregnancy is by measuring the crown-rump length (CRL)
- bi-parietal diameter (BPD) and femoral length (FL) are less accurate when used alone
- usually CRL, BPD and FL are combined with other clinical factors to estimate gestational age
- this is important in dating a pregnancy** and **interpreting serum markers
How is nuchal translucency measured?
When is it measured and why is it significant?
- this measures the thickness of fluid present in the subcutaneous tissue in the nuchal region
- it is performed between 11-14 weeks, but usually at 12 weeks alongside the combined test
- nuchal translucency > 3mm** is strongly associated with **cardiovascular system defects and chromosomal abnormalities
- it is particularly indicative of Down’s syndrome
What are the 2 types of diagnostic tests and why are they performed?
What must the mother be aware of before agreeing to these tests?
- chorionic villus sampling (CVS)
- amniocentesis
- they are performed to confirm any suspected abnormality
- diagnostic tests are INVASIVE procedures and carry a risk of damage to the foetus and foetal death
When are diagnostic tests routinely offered?
- diagnostic tests are routinely offered regardless of screening tests when:
- mother is >35 years of age
- previous child with a congenital abnormality
- one of the parents has a chromosomal disorder
- they are also offered when a screening test is suggestive of a congenital abnormality being present