# Liquid biopsy

A liquid biopsy, also known as fluid biopsy or fluid phase biopsy, is the sampling and analysis of non-solid biological tissue, primarily blood. Like traditional biopsy, this type of technique is mainly used as a diagnostic and monitoring tool for diseases such as cancer, with the added benefit of being largely non-invasive. Liquid biopsies may also be used to validate the efficiency of a cancer treatment drug by taking multiple samples in the span of a few weeks. The technology may also prove beneficial for patients after treatment to monitor relapse.
The clinical implementation of liquid biopsies is not yet widespread but is becoming standard of care in some areas.
Liquid biopsy refers to the molecular analysis in biological fluids of nucleic acids, subcellular structures, especially exosomes, and, in the context of cancer, circulating tumor cells.

## Types

There are several types of liquid biopsy methods; method selection depends on the condition that is being studied.
| Disease | Tissue sampled | Sampling procedure | Invasiveness | Substance isolated | Isolation and detection method | Analysis |
| Cancer (various) | Blood | Phlebotomy | Minimally invasive | Circulating tumor cells (CTCs) | Various (e.g. CellSearch, RosetteStep, Dynabeads) | Flow cytometry, nucleic acid extraction, immunocytochemistry, functional assays |
| Cancer (various) | Blood | Phlebotomy | Minimally invasive | Circulating tumor DNA (ctDNA) | DNA extraction | Next-generation sequencing |
| Urothelial carcinoma | Urine | Urine collection | Non-invasive | Urinary tumor DNA (utDNA) | DNA extraction | Next-generation sequencing |
| Non-urological cancers | Urine | Urine collection | Non-invasive | Urine proteins, metabolites | HPLC-MS | Proteomics, metabolomics |
| Bladder and prostate cancer | Urine | Urine collection | Non-invasive | Exfoliated cancer cells | Urinalysis | Fluorescence in situ hybridization |
| Heart attack | Blood | Phlebotomy | Minimally invasive | Circulating endothelial cells (CECs) | Various (e.g. CellSearch, HD-CEC) | Flow cytometry |
| Neurological diseases | Cerebrospinal fluid | Lumbar puncture | Invasive | CSF proteins, nucleic acids | Various | ELISA, multiplex assay, next-generation sequencing |
| Prenatal diagnosis | Blood (maternal) | Phlebotomy | Minimally invasive | Cell-free fetal DNA (cffDNA) | DNA extraction | Karyotyping, fluorescent in situ hybridization |
| Prenatal diagnosis | Blood (maternal) | Phlebotomy | Minimally invasive | Fetal cells in maternal blood (FCMB) | Flow cytometry | Karyotyping, fluorescent in situ hybridization |
| Prenatal diagnosis | Blood (umbilical cord) | Cordocentesis | Invasive | Umbilical blood cells and molecules | Various | Karyotyping, blood typing, blood tests, Kleihauer–Betke test, flow cytometry |
| Prenatal diagnosis | Amniotic fluid | Amniocentesis | Invasive | Amniotic fluid cells and molecules | Various | Karyotyping, blood typing, L/S ratio, S/A ratio |
A wide variety of biomarkers may be studied to detect or monitor other diseases. For example, isolation of protoporphyrin IX from blood samples can be used as a diagnostic tool for atherosclerosis. Cancer biomarkers in the blood include PSA (prostate cancer), CA19-9 (pancreatic cancer) and CA-125 (ovarian cancer).

## Mechanism

Circulating tumor DNA (ctDNA) refers to DNA released by cancerous cells into the blood stream. Cancer mutations in ctDNA mirror those found in traditional tumor biopsies, which allows them to be used as molecular biomarkers to track the disease. These tests can have sensitive limits of detection, allowing monitoring of minimal residual disease after treatment. Scientists can purify and analyze ctDNA using next-generation sequencing (NGS) or PCR-based methods such as digital PCR. NGS-based methods provide a comprehensive view of a cancer’s genetic makeup and is especially useful in diagnosis while digital PCR offers a more targeted approach especially well-suited for detecting minimal residual disease and for monitoring treatment response and disease progression. Recent progress in epigenetics has expanded the use of liquid biopsy for the detection of early-stage cancers, including by approaches such as Cancer Likelihood in Plasma (CLiP) .
Liquid biopsies can detect changes in tumor burden months or years before conventional imaging tests can, making them suitable for early tumor detection, monitoring, and detection of resistance mutations. The increase in the adoption of NGS in various research fields, advancement in NGS, and increase in the adoption of personalized medicine are expected to drive growth in the global liquid biopsy market.

## Clinical application

In cancer, liquid biopsy can be used for either multi-cancer screening tests, when solid tumor biopsies are not possible, to compare different treatments as part of clinical trials, to inform decisions for doctors/patients on which precision medicine treatment to select, and for minimal residual disease detection (disease monitoring). Liquid biopsy of circulating tumor DNA for EGFR-mutated lung cancer is approved by the FDA.
The CellSearch method for enumeration of circulating tumor cells in metastatic breast, metastatic colon, and metastatic prostate cancer has been validated and approved by the FDA as a useful prognostic method.