Lecture 18- Testing for genetic disorders Flashcards Preview

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Purpose of Prenatal testing and screening

Often done if there is a concern of risk
Guides families in decisions around the pregnancy
-potential termination?
-Medical & psychological support


What is Prenatal Testing?

Screening tests and diagnostic test


Difference between screening and diagnostic tests

-assess RISK but not a definitive diagnostic test
-non-invasive, low risk
eg) maternal serum testing for downs syndrome

Diagnostic Tests
-Test for specific chromosomal or genetic abnormality (usually via fetal cells/ DNA)
-invasive test, carries some risk
eg) gene mutation analysis for cystic fibrosis


What are the clinical indications to do prenatal testing for Chromosomal or single gene disorders

Single Gene disorders:
-family history
-one parent has AD disorder
-if both parents carriers
-X linked
(risk prediction done from analysis of pedigree, may lead to specific diagnostic test).

Chromosomal Disorders
-older mother pregnant
-previous child with significant disorder
-chemical/toxic disorders
-Abnormal screening test result


Screening Stratagies

-Ultrasound scan of pregnancy
-Nuchal Translucency
-Maternal Serum Testing


Screening Ultrasound

8 wks : check dates, confirm # babies
18wks: detailed scan of anomalies can be done

Safe, no risk to mother or fetus


Nuchal Translucency

collection/thickness of fluid under the skin at the back of your baby's neck.
Measure by ultrasound ~10-13 wks
Noninvasive test
(combined with maternal serum testing to increase sensitivity and specificity for downs syndrome screening)


Maternal Serum Testing

Blood test
Performed ~15-17 wks
-testing for 2-4 feto-placental proteins1


Risk prediction for Down syndrome

Screening in 2nd trimester by --maternal age and maternal serum can identify 50-75% trisomy 21 pregnancies with a false + rate of 5%

-Fetal NT in 1st and maternal serum in the 2nd reported a detection rate of 85-90% trisomy 21 with a false + rate of 5%


Types of Diagnostic Tests

Access to fetal cells
May lead to specific diagnostic test
1) amniocentesis (weeks to culture)
2) Chronic villus test (hours)

These all take 3-4 days to do this testing

also percutaneous umbilical cord blood sampling
-fetoscopy & umbilical cord blood sampling
(done later, very rare to do, high chance of miscarriage)


Chorionic Villus Sampling

cells from developing placenta taken and tested
Preferred as done earlier ~10-12 wks
-obtain part of CV (fetal cells), and analyse chromosomes or DNA
-risk of miscarriage



Needle injected into uterus, collects amniotic fluid that contains fetal cells
-may need to culture cells before analysis
-Miscarriage risk ~1%



cell free fetal DNA in the maternal plasma
-non-invsive screening test for anueploidies
-still have to confirm with a diagnostic test
-Not routinely available in NZ
Detection Rate: 99%
False +: 0.9%


Issues with new screening test

Not comrehensive testing
-only for common chromosome disorders
-single gene disorders (need to know mutation)
-risk of error

Risk (small) to fetus

Not all think its morally/ethically acceptable


Benefits when testing prenatal testing

Reassurance when results are normal
Advance warning for medical team
psychological preparation for babies arrival
-info, is termination an option?


Pre-Impantation Genetic diagnosis (PGD)

Uses IVF and tests for the genetic disorder before fertilised eggs are implanted.
-Eggs fertilised 'in-vitro', tested in lab to determine if they are genetically affected.
-Non-affected eggs implanted

Expensive, technically difficult


Genetic Counselling

Essential part of the process and car of the patients
-Provide up to date info & support