Lecture 20: Testing for genetic disorders Flashcards
What is the function of prenatal screening and diagnosis?
Screening for and the diagnosis of genetic and congenital disorders in established pregnancies
Why do prenatal testing?
- Guides decision making
- Might involve termination -> Medical and psychological support
- Majority of testing normal, provides reassurance
What are screening tests?
Assess risk but not definitive diagnostic test i.e maternal serum testing for down syndrome.
- Non-invasive, low risk tests
What are diagnostic tests?
Where test for a specific chromosomal or genetic abnormality i.e gene mutation analysis for cystic fibrosis
- Invasive, carries risk
What are some clinical indications for prenatal testing?
- (Serious) Single gene disorder
- Considered at risk for constitutional chromosomal disorder
What about single gene disorders would prompt prenatal testing?
- Family history
- Both parents carriers of autosomal recessive disorder
- Female carrier of x linked haemophilia
- One parent has an autosomal dominant disorder
What makes a couple considered to be at risk of a constitutional chromosomal disorder?
- Older mother pregnancy
- Previous child with significant developmental or other disorder
- Exposure to chemical or other toxic agent
- Abnormal screening test result
What are the prenatal screening stratagies?
Ultrasound scan of pregnancy
Maternal serum testing
Nuchal translucency
Pedigree tree (single gene disorders)
What are prenatal diagnostic stratagies?
CVS; Chronic villus sampling
Amniocentesis
Fetoscopy
Umbilical cord blood sampling
i.e karyotyping
Whats the value of ultrasound scan in pregnancy?
- 8 Weeks confirm number babies
- 18 weeks scan for anomalies ie anencephaly, cardiac abnormalities
- Nuchal cord translucency (down syndrome screening)
What does the maternal serum indicate?
Trimester 1 blood test; With nuchal translucency can indicate down syndrome
Trimester 2 is just serum alone.
How effective is screening for trisomy 21? using US and maternal serum?
Maternal serum alone; 50-75%, false positive ~5%
Maternal serum and nuchal translucency, 85-95% with false positive of 5%
Can lead to diagnostic test
What does CVS and amniocentesis allow?
Obtain fetal cells for further analysis i.e Karyotype, FISH studies or single gene mutation analysis
Write some notes on CVS;
CV sampling
- 10-12 weeks of pregnancy
- Obtain fetal cells - Analyse chromosomes or DNA
- Risk of miscarriage <1%
Write some notes on amniocentesis;
- 15-16 weeks pregnancy
- Collect fluid which contains fetal cells
- May need to culture cells before analysis
- Miscarriage risk <1%
