Placental Cell-Free DNA

1. See Also
   1. Antenatal Screening
   2. Aneuploidy Screening
   3. First Trimester Education
   4. Second Trimester Education
   5. Pregnancy Risk Assessment
   6. Trisomy Findings on Fetal Ultrasound
   7. Nuchal Translucency
   8. Second Trimester Ultrasound
2. Indications
   1. Congenital defect screening in pregnancy (e.g. Trisomy 21, Trisomy 18, Trisomy 13, Sex ChromosomeAneuploidy) AND
   2. High risk women for child with congenital defects
      1. Maternal Age 35 years or older
      2. Ultrasound findings at higher risk of Aneuploidy
      3. Prior chromosomal defect in pregnancy (esp. Trisomy 21, Trisomy 18, Trisomy 13)
      4. Positive results on First Trimester Combined Screening or Second Trimester Quad Maternal Screen Panel
      5. (2015) Am J Obstet Gynecol 212(6):711-6 [PubMed]
3. Mechanism
   1. Placental DNA fragments circulate in maternal blood as Fetal Cell-Free DNA
   2. Maternal serum is collected and Fetal DNA is isolated and amplified
   3. Fetal DNA distribution from each Chromosome is quantified, therefore able to predict Aneuploidy
4. Protocol
   1. Obtain sample after 10 weeks gestation
      1. Testing earlier than 10 weeks may obtain too little fetal DNA to characterize accurately (see low fetal fraction below)
   2. Confirm positive results with invasive testing (Chorionic Villus Sampling or Amniocentesis)
5. Efficacy: Trisomy 21
   1. Test Sensitivity: 99.7%
   2. Test Sensitivity in Twin Pregnancy: 99%
   3. Negative Predictive Value >99% regardless of age
   4. False Positive Rate is 0.04% (contrast with rates of 3-5% for Quad Screen)
   5. Positive Predictive Value (PPV) changes significantly with advancing maternal age
      1. Age 20 years old PPV: 68% (Trisomy 21Prevalence 1 per 1177)
      2. Age 25 years old PPV: 71% (Trisomy 21Prevalence 1 per 1040)
      3. Age 30 years old PPV: 78% (Trisomy 21Prevalence 1 per 700)
      4. Age 35 years old PPV: 89% (Trisomy 21Prevalence 1 per 296)
      5. Age 40 years old PPV: 97% (Trisomy 21Prevalence 1 per 86)
6. Efficacy: Trisomy 18
   1. Test Sensitivity: 78.9 to 97.9%
   2. Test Sensitivity in Twin Pregnancy: 85%
   3. False Positive Rate: 0.01 to 0.04%
7. Causes: Low Fetal Fraction (results too low to report)
   1. Testing earlier than 10 weeks gestation
   2. Obesity
      1. Low Fetal Fraction in 10% who weigh >100 kg
      2. Low Fetal Fraction in 51% who weigh >160 kg
8. Causes: Abnormal Cell-Free DNA with Normal Invasive Testing
   1. Placental Mosaicism
   2. Maternal Aneuploidy
   3. Occult Maternal Malignancy
9. References
   1. Gil (2017) Ultrasound Obstet Gynecol 50(3): 302-14 [PubMed]
   2. LeFevre (2020) Am Fam Physician 101(8):481-8 [PubMed]
   3. Liao (2017) Prenat Diagn 37(9): 874-82 [PubMed]
   4. Norton (2015) N Engl J Med 372(17): 1589-97 [PubMed]