Perinatal Diagnosis

Question 1

Which of these two have the lowest false positive rate. Amniocentesis verses Chorionic Villus Sampling

Answer 1

Answer is Amniocentesis.

CVS has a 1-2% rate of confined placental mosaicism which would contribute to the false positive rate.

Question 2

A carrier of cystic fibrosis with a Caucasian husband with an unknown carrier status has what percent likelihood of having a child with cystic fibrosis

Answer 2

CF is autosomal recessive affecting chromosome 7 - specifically the CF conductance transmembrane regulator -CFTR gene.

Carrier risk is 50%
General population risk is 4%

0.50.50.04= 0.01

Question 3

A carrier of cystic fibrosis with a Caucasian husband with a negative carrier screen has what percent likelihood of having a child with cystic fibrosis

Answer 3

One would need to discover the sensitivity of the particular panel. For example if the panel can detect 80% of mutations then his residual risk of being a carrier is general carrier rate of 4% multiplied by 20% (0.040.2)= 0.008. Therefore the calculation would then be,
0.50.008*0.5= 0.002