Cell Free DNA (cfDNA) Prenatal Screening

1. INTRODUCTION

• Definition: A non-invasive prenatal screening (NIPS) test that analyzes circulating cell-free DNA in maternal blood to estimate the risk of fetal chromosomal aneuploidies.
• Source: cffDNA (cell-free fetal DNA) originates from the apoptosis of placental trophoblasts (not the fetus directly).
• Clearance: Rapidly cleared from maternal circulation (undetectable 2 hours postpartum).

2. TIMING & PREREQUISITES

• Timing: Can be performed from 9–10 weeks of gestation onwards.
• Fetal Fraction (FF): The proportion of cffDNA in maternal plasma.
  • Requires >4% FF for a reliable result.
  • Low FF causes: Maternal obesity (dilution), early gestation, LMWH use.

3. INDICATIONS (ACOG Guidelines)

Initially restricted to high-risk pregnancies, now recommended as an option for all pregnant women regardless of risk status.

• High-Risk Indications:
  • Advanced Maternal Age (>35 years).
  • Abnormal serum screen (Positive Combined/Quad test).
  • Ultrasound soft markers.
  • Previous history of trisomy.

4. SCOPE OF DETECTION

1. Core Panel: Trisomy 21 (Down), Trisomy 18 (Edwards), Trisomy 13 (Patau).
2. Sex Chromosome Aneuploidies (SCA): Turner (45,X), Klinefelter (47,XXY).
3. Expanded Panels (Variable):
   • Microdeletions (e.g., 22q11.2 deletion / DiGeorge).
   • Some single-gene disorders (e.g., Achondroplasia).

5. PERFORMANCE METRICS

• Sensitivity: Highest for Down Syndrome (>99%). Slightly lower for T18 and T13.
• False Positive Rate: Very low (<0.1%).
• Superiority: Outperforms standard serum screening (Quad/Combined test) in sensitivity and specificity.

6. LIMITATIONS & PITFALLS (Crucial for Exam)

• Screening ONLY: It is NOT diagnostic. A "High Risk" result must be confirmed by invasive testing (Amniocentesis/CVS) before termination of pregnancy.
• False Positives:
  • Confined Placental Mosaicism (CPM): DNA is placental; if placenta is abnormal but fetus is normal, NIPT is false positive.
  • Vanishing Twin: DNA from a demised co-twin.
  • Maternal Malignancy: Tumors shed cell-free DNA.
• Test Failure: Usually due to low Fetal Fraction (high BMI).

7. SUMMARY

• Gold Standard Screening: Most accurate screening method for Trisomy 21.
• Safety: Zero risk of miscarriage (non-invasive).
• Limitation: Does not detect NTDs (still requires USG/AFP) and is cost-prohibitive in some settings.