Cell-free DNA (cfDNA)
Short pieces of DNA floating in the blood, shed by dying cells all over the body. Most is from normal cells; the small tumour-derived fraction (ctDNA) is what liquid biopsies fish out.
A tube of blood contains nanograms of cell-free DNA, of which tumour DNA may be 0.01-10%. Assays detect it by deep sequencing with error correction (Guardant360, FoundationOne Liquid, MSK-ACCESS), by tumour-informed panels built from the patient's own tumour mutations (Signatera, RaDaR), or by methylation and fragment-size patterns that also reveal the tissue of origin (Galleri, fragmentomics). Uses span genotyping when tissue is scarce, resistance monitoring, MRD after surgery and multi-cancer screening. Pitfalls are CHIP-derived variants, low shedding by some tumours (brain, kidney) and pre-analytic handling, hence dedicated collection tubes.
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