Antenatal Screening Flashcards
Define screening
The UK National Screening Committee:
Screening is a process of identifying apparently healthy individuals who may be at an 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 are the WHO/modified Wilson criteria for a screening programme?
IATROGENIC:
Important – the condition should be an important one
Acceptable treatment for the disease
Treatment and diagnostic facilities should be available
Recognisable at an early stage of symptoms
Opinions on who to treat as patients must be agreed
Guaranteed safety e.g. low radiation exposure
Examination must be acceptable by the patient
Natural history of the disease must be known
Inexpensive test
Continuous screening i.e. not a one-off
What is required of health professionals before screening commences?
The condition that is being screened for
When + how the test will be carried out
How reliable the test is
Different possible results and their meanings
Options if the test is positive
What is a detection rate? How does this relate to false positive and false negative rates?
Detection rate: proportion of affected individuals who will be identified by screening test
False positive rate: proportion of unaffected individuals with a higher risk/screen positive result
False negative rate: proportion of affected individuals with a low risk/screen negative result
What antenatal screening programmes exist?
Fetal Anomaly Screening Programme
Infectious Diseases Screening Programme
Sickle Cell and Thalassaemia Screening Programme
What newborn screening programmes exist?
New-born blood spot screening programme
New-born hearing programme
New-born AND 6 – 8 week infant physical examination screening programme
What are the features of the Fetal Anomaly Screening Programme? (FASP)
Screens for the three common trisomies: Down’s (T21), Edward’s (T18), Patau’s (T13)
Combined or Quadruple tests
What is the combined test on the FASP?
Used in 1st trimester
Nuchal translucency scan:
Offered if crown-rump length between 45-84mm Between 11+2 - 14+1wks
INCREASED NUCHAL THICKNESS
Blood sample taken on same day (before 14+1):
Beta HCG - INCREASED
PAPP-A - DECREASED
Detection rate = 85%,
2 risk results:
T21 + T18/T13
Cut off for Dx testing (amnioentesis/CVS) is 1/150
What is the quadruple test on the FASP?
Given if between 14-20wks gestation
Offered if combined not possible e.g. late booker, nuchal translucency not obtained
Offered when head circumference 101mm-173mm between 14+2 - 20+0wks
Uses serum markers only: Inhibin A - INCREASED total Beta HCG - INCREASED Alpha Feto Protein - DECREASED Oesrtiol - DECREASED
Scan measurements + mothers DOB + weight + serum markers adjusted for ethnicity/smoking/DM = risk score
Detection rate >75%
What happens when there is a strong positive screen on FASP?
Fetal medicine unity appointment within 3 days
Offered Dx tests = CVS or amniocentesis - both have associated risks
Option of non-invasive testing (NIPT) in private sector
What are amniocentesis and CVS and the risks associated with them?
Amniocentesis = extraction and analysis of amniotic fluid (mostly foetal urine) for foetal skin cells that can then be karyotyped to look for the relevant trisomies; only possible from week 15-20
Chorionic villus sampling (CVS) - takes a sample of the placenta though abdomen or cervix, takes placental cells which will be identical to the baby for karyotyping ; possible from earlier date of 10-13wks
Results usually take 2wks
As both are invasive, both pose a risk of miscarriage to the foetus = 1/100 for both tests (1/50 if twins)
What is NIPT?
Non-invasive, maternal blood sample, this time looking for fragments of foetal DNA
Still a screening, not a diagnostic test
Can be done from 10wks of pregnancy - only private currently
Predicts foetal gender and risk of T21, 18, 13
99% detection rate; 0.1% false positives
When do women have ultrasound scans as part of the FASP?
Early USS - (8) 10-14wks - mainly for dating, checking for multiple pregnancy, confirming viability and elucidating possible abnormality (e.g. exomphalus, increased nuchal translucency)
Structural anomaly scan - 18-20+6 weeks - anencephaly, open spina bifda, cleft lip, diaphragmatic hernia, gastroschisis, exomphalos, cardiac abnormalities, renal agenesis, lethal skeletal dysplasia, T12 T18 - if positive findings, relevant specialists get involved asap
Timings allow for further diagnostic tests to allow women enough time to make decisions regarding progression with pregnancy
What screening for infectious diseases occurs during pregnancy?
HIV:
Treatment improves mother prognosis and reduces foetal transmission
Hep B:
Looks for current infection (chronic), vaccination of mother and reduced risk fo transmission to baby
Syphilis:
Current infection, treatment to prevent congenital syphilis and complications for mother
Offered to all women in any stage of pregnancy when they present to care- blood samples should be taken at the earliest opportunity
What are the haemoglobinopathies screened for?
Alpha and beta thalassaemias - 214,000 carriers:700 affected; alpha thalassaemia major = incompatible with extrauterine life; beta thalassaemia major = life threatening anaemia (blood transfusions every 4-6wks and iron cheltion 5-7x weekly)
Sickle cell disease - 240,000 carriers:12,500 affected in UK
All recessive inheritance = 1/4 chance of disease of both parents are carriers
