In a traditional biopsy, a small piece of tissue is taken from a living body and analysed for disease. This is invasive and not undertaken lightly. In the case of suspected cancer, by the time the doctor feels that a biopsy is required, the cancer has progressed some way already.
The liquid biopsy has the advantage that the sample is a blood sample, so fairly easy to extract. There is research underway to show that such a blood sample can reveal whether a person has cancer or not at a very early stage. Such a test, when developed, could be a gamechanger.