A liquid biopsy primarily detects circulating tumour DNA, small fragments of genetic material released into the blood when tumours cells die or break down. It can also detect circulating tumours cells (whole cancer cells that have broken away from the tumours and entered the bloodstream) and, in some tests, tumour-derived exosomes (tiny particles released by cancer cells that carry genetic and protein material).

From these components, a liquid biopsy can identify:
*Specific gene mutations driving the cancer, such as changes in EGFR, KRAS, or BRAF
*Gene amplifications, such as HER2
*Resistance mutations that develop after a patient starts targeted therapy
*The overall amount of tumour DNA in the blood, which can reflect disease burden
*Early signs of cancer recurrence, sometimes before it is visible on imaging scans