How does cell-free fetal DNA testing work

How cell-free fetal DNA testing works

Cell-free fetal DNA (cffDNA) testing is a non-invasive prenatal screening method that analyzes fetal DNA circulating in the mother’s blood to assess the risk of certain chromosomal abnormalities.

Here’s how it works:

How Cell-Free Fetal DNA Testing Works

1. Sample Collection

Timing: The test can be performed as early as 10 weeks into the pregnancy and continues until delivery.

Blood Draw: A blood sample is taken from the pregnant woman, which contains both maternal and fetal DNA. Approximately 11-13% of the cell-free DNA in maternal blood is of fetal origin.

2. DNA Analysis

Separation: The laboratory isolates the cffDNA from the maternal blood sample. This fetal DNA is typically fragmented into small pieces, around 200 base pairs in length, making it distinguishable from maternal DNA.

Sequencing Techniques: Various sequencing methods are employed to analyze the cffDNA, including:

•  Massively Parallel Shotgun Sequencing (MPSS)
• Targeted Massive Parallel Sequencing (t-MPS)
• Single Nucleotide Polymorphism (SNP) Analysis.

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3. Screening for Chromosomal Abnormalities

The primary aim of cffDNA testing is to screen for common chromosomal abnormalities, notably:

• Trisomy 21 (Down syndrome)
• Trisomy 18 (Edwards syndrome)
• Trisomy 13 (Patau syndrome) 

The test detects an abnormal amount of DNA from these chromosomes in the maternal blood, as fetuses with these conditions may release different quantities of specific chromosome fragments into the bloodstream.

4. Interpretation of Results

Results are typically categorized as either positive or negative:

•  A positive result indicates a higher risk of the specified chromosomal abnormalities, warranting further diagnostic testing such as amniocentesis or chorionic villus sampling (CVS) for confirmation.
• A negative result suggests a lower risk, but it does not completely rule out the possibility of abnormalities due to limitations in sensitivity and specificity.

5. Advantages and Limitations

Advantages:

• Non-invasive and carries no risk to the fetus.
• High accuracy rates, with detection rates exceeding 99% for conditions like Down syndrome.

Limitations:

• It is a screening test, not a diagnostic test; thus, positive results require follow-up with diagnostic procedures.
• There can be false positives and false negatives, particularly influenced by factors such as maternal weight and gestational age.

In summary, cell-free fetal DNA testing provides an innovative approach to prenatal screening that enhances early detection of potential genetic disorders while minimising risks associated with invasive testing methods.

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