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A liquid biopsy is a blood test that looks for fragments of genetic material and cells that a tumour has shed into the bloodstream. Instead of surgically removing a piece of the tumour itself, doctors draw a blood sample and analyse it for circulating tumour DNA, circulating tumour cells, and other tumour-related material floating in the blood.
A standard tissue biopsy involves a needle, surgical procedure, or endoscopic sampling to physically remove a piece of the tumour for laboratory examination. This gives a direct, detailed look at the tumour tissue but is invasive, carries procedural risk, and captures only the specific spot that was sampled.
A liquid biopsy is minimally invasive, requiring only a blood draw, and because tumours shed material from multiple sites into the bloodstream, it can reflect the genetic makeup of the cancer more broadly rather than just one location.
Blood is the standard sample because it’s easily accessible and tumours reliably shed genetic material, cells, and other components into the bloodstream, where they can be captured as circulating tumour DNA, circulating tumour cells, or tumour-derived exosomes. The term “liquid biopsy” broadly refers to testing any bodily fluid for tumour-derived material, but in clinical practice today, blood-based liquid biopsy is what is most validated, most widely offered, and most often meant when the term is used without further specification.
Other body fluids can also be used, depending on where the cancer is located:
*Urine is used in some bladder and prostate cancer applications, since tumour material from the urinary tract can shed directly into urine.
*Cerebrospinal fluid is used for brain tumours and cancers that have spread to the central nervous system, since brain tumour material does not cross into the bloodstream as reliably as it does with other organs.
*Pleural or peritoneal fluid, the fluid build-ups around the lungs or abdomen in advanced cancer, which can be tested when a patient already has fluid drained for symptom relief.
*Saliva is being explored, particularly for head and neck cancers, though less established in routine clinical use.
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