Understanding Fetal Fraction 8 on NIPT
A fetal fraction of 8 on Non-Invasive Prenatal Testing (NIPT) indicates an adequate amount of fetal DNA in the maternal blood sample for reliable testing, which is well above the minimum threshold typically required for accurate results. 1, 2
What is Fetal Fraction?
- Fetal fraction refers to the proportion of cell-free DNA in maternal blood that originates from the placenta (presumed to represent the fetus) 1, 3
- It is a critical quality control component for NIPT results 4
- Fetal fraction is typically expressed as a percentage of the total cell-free DNA in maternal plasma 5
Interpretation of Fetal Fraction 8
- A fetal fraction of 8% is considered adequate for reliable NIPT analysis 2, 5
- Most NIPT platforms require a minimum fetal fraction of approximately 4% to provide results 2
- With a fetal fraction of 8%, the test is unlikely to result in a "no-call" result, which occurs in approximately 1% of NIPT samples 1
Factors That Affect Fetal Fraction
- Maternal factors: High BMI is strongly associated with lower fetal fraction; use of certain anticoagulants and presence of autoimmune disorders can also reduce fetal fraction 2, 1
- Gestational age: Fetal fraction typically increases with gestational age, though there may be a drop around 15 weeks 6, 2
- Fetal factors: Certain chromosomal abnormalities, particularly trisomies 13 and 18, can be associated with lower fetal fraction 2, 3
- Multiple gestations: The presence of multiple fetuses can affect fetal fraction measurements 2, 5
Clinical Implications
- A fetal fraction of 8% suggests that the NIPT result is likely to be reliable 1, 2
- Higher fetal fraction is associated with greater test accuracy and lower likelihood of test failure 4
- Recent evidence suggests that low fetal fraction (which is not the case with a value of 8%) may be associated with adverse pregnancy outcomes, particularly hypertensive disorders of pregnancy 7
When to Consider Additional Testing
- If NIPT results are positive despite adequate fetal fraction, diagnostic testing (amniocentesis or CVS) is recommended for confirmation 3, 1
- Even with adequate fetal fraction, NIPT remains a screening test, not a diagnostic test 3, 5
- The positive predictive value of NIPT for trisomy 21 is 50-95%, which is significantly higher than traditional screening methods (2.2-3.6%) 1
Important Considerations
- NIPT with adequate fetal fraction significantly reduces the need for invasive diagnostic procedures compared to traditional screening methods 1
- Pre-test and post-test genetic counseling are integral aspects of NIPT, regardless of fetal fraction 1, 3
- NIPT results should always be interpreted in the context of other clinical findings, including ultrasound findings 1, 3