Question 2

A 36 yo G3P2 undergoes cell free DNA screening at 11 weeks gestation. The results are consistent with trisomy 21. What is the most appropriate next test?

Answer Choices:

Chorionic villus sampling

First-trimester serum analyte screening

Repeat cell free DNA screening

Ultrasound of the ductus venosus

Parental karyotype

Correct Answer:

Chorionic villus sampling

Explanation:

  • Cell free DNA is primarily derived from the placenta

  • Sensitivity for trisomy 21, 18, 13 and the sex chromosomes is high, but the positive predictive value is lower and depends on maternal age

  • Accuracy of cell free DNA is limited in multiples

  • A diagnostic testing is recommended after positive cell free DNA screening

  • Options include chorionic villus sampling or amniocentesis

  • Amniocentesis usually done after 16 weeks, Chorionic villus sampling is done between 11-14 weeks

  • Serum analytes do not offer any additional benefit after cell free DNA

  • Repeat cell free DNA screening is not recommended

  • Paternal karyotypes are recommended only if diagnostic testing (amniocentesis or CVS) show a translocation type of trisomy which has a risk of recurrence