Plasma cell-free DNA (cfDNA) is widely used in tumor liquid biopsy, prenatal testing, transplant monitoring, and infectious pathogen surveillance. Because cfDNA is low in abundance, short-fragmented, and easily affected by pre-analytical factors, the extraction step directly determines the sensitivity and reproducibility of downstream qPCR/ddPCR/NGS.
I. What is Cell-Free DNA in Plasma?
Plasma cell-free DNA (cfDNA) refers to DNA that exists in the bloodstream but is not contained within cells. Its main sources include:① DNA released from white blood cells during apoptosis or cell lysis;② DNA originating from necrotic tissues, particularly tumor tissues that release genetic material into the blood;③ DNA from pathogens such as viruses or bacteria during infection.Today, cfDNA is recognized as a novel biomarker and is widely used in various clinical applications, including prenatal screening and diagnosis, cancer detection, monitoring of organ transplant rejection, and infection assessment.
II. Pre-analytical Essentials
cfDNA is a mixture of extracellular DNA originating from multiple systems, among which tumor-derived ctDNA often exists at an extremely low variant allele frequency (VAF). To achieve reproducible and traceable analytical performance, pre-analytics and extraction chemistry must be jointly optimized:
vBlood collection and processing time: Prefer EDTA or cfDNA stabilization tubes; promptly perform double centrifugation after collection to remove cells and nuclear debris, avoiding gDNA contamination caused by leukocyte lysis.
vStorage and transport: Plasma should be stored at low temperature and avoid repeated freeze–thaw; aliquot when necessary.
vContamination control: Set up blank controls (no template, negative plasma) to monitor background from environment and plastic consumables.
III. Core Dimensions Affecting cfDNA Extraction Performance
1.Short-fragment recovery (~150–200 bp)
vDirectly determines ctDNA detection sensitivity and the number of effective molecules for methylation/ultra-deep sequencing.
vPay attention to the recovery curve and fragment bias for ultra-short fragments <150 bp.
2.Input volume and elution volume
vLarge input (commonly 2–5 mL plasma) + low elution volume (20–100 µL) can significantly lower the detection limit.
vThe composition and volume of the elution buffer will affect inhibition of downstream enzymatic reactions and library construction efficiency.
3.Throughput and automation capability
vMagnetic-bead methods are easier to run on platforms such as KingFisher and Maxwell; column methods have traditional advantages in large input/low elution.
vPay attention to disposable consumables, hands-on time, and intra/inter-batch coefficient of variation (CV).
4.Downstream compatibility
vTolerance for workflows such as NGS (including capture/amplification library construction), ddPCR/qPCR, methylation/bisulfite conversion.
vChoose processes with “low residual inhibitors and thorough removal of salts/ethanol.”
5.Matrix adaptability
vTolerance to hemolytic, lipemic, and icteric samples, and recovery strategies (extend protease digestion, increase washing steps, etc.).
IV. cfDNA Extraction Methods
3.1 Silica membrane (spin column/vacuum column)
Principle: Under high-salt/low-pH conditions, DNA adsorbs to the silica membrane and is released under low-salt conditions during elution.
Advantages: Mature and stable for short-fragment recovery; supports large input and low-volume elution; straightforward workflow.
Limitations: Limited for high throughput and unattended automation (can be partially alleviated with multi-manifold vacuum devices).
Typical applications: High-sensitivity ctDNA, methylation assays, miRNA/cfRNA (requires dedicated workflows), etc.
3.2 Magnetic beads (solid-phase adsorption in solution)
Principle: DNA adsorption onto magnetic beads is regulated by polyethylene glycol (PEG) and salt.
Advantages: Suitable for automated high throughput; short operation time and reduced operator variability.
Key points: The bead/PEG/salt formulation determines fragment selectivity; pay attention to the recovery curve for 100–200 bp fragments.
Typical applications: Routine liquid-biopsy screening, high-throughput extraction for cohort samples, end-to-end automated pipelines.
V. Frequently Asked Questions (FAQ)
Q1: Total cfDNA is low and the TapeStation shows essentially no peak?
Check delays after collection and hemolysis; try increasing input volume and reducing elution volume; extend protease digestion and lysis time.
Q2: Library construction fails or yields are low?
Determine whether salt/ethanol residue is present: appropriately extend on-column drying or bead air-drying; switch to EDTA-free elution.
Q3: Background gDNA is high with smeared fragments?
Strengthen double centrifugation and avoid disturbing the supernatant during transfer; increase deproteinization and washing stringency; if necessary, add a 0.8× bead cleanup before library prep to retain high-molecular-weight DNA.
Q4: Detection at extremely low VAF is unstable?
Focus on optimizing short-fragment recovery and elution volume; increase input plasma volume; use UMI library construction and ddPCR verification.
VI. Aladdin Typical Products
ØFree DNA extraction magnetic beads (Cat. No. F1456624): Magnetic solid-phase carrier beads designed specifically for cfDNA extraction. The special manufacturing process provides stronger magnetism and improved capture of short fragments. Surface silanol groups bind nucleic acids specifically under high-salt and low-pH conditions, improving genomic purity and yield. When paired with automated nucleic acid extraction platforms, the entire extraction process becomes more efficient and safer, and is widely applicable in life-science research and applications.
ØCWhipro Circulating Nucleic Acid Kit (Cat. No. C665709): This kit uses spin adsorption columns that specifically bind nucleic acids and a unique buffer system. After sample lysis, cfDNA binds to the silica membrane under high-salt conditions and is eluted from the silica membrane under low-salt and high-pH conditions. This product can process 0.1–1 mL liquid samples, and the supplied high-efficiency micro spin column allows elution volumes as low as 20 μL.
ØCWhipro Circulating DNA Midi Kit (Cat. No. C665715): This kit uses a vacuum method and comes with extension tubes, enabling processing of up to 5 mL samples. The purified DNA has high yield and good quality, maximizing removal of proteins, pigments, lipids, and other inhibitory contaminants. The purified cfDNA is stable and reliable, and can be directly used for PCR, qPCR, and next-generation sequencing and other molecular biology experiments.
Aladdin: https://www.aladdinsci.com/