DNA from tumor cells is thought to enter (https://www.mycancergenome.org/content/page/circulating-dna/) the bloodstream in variety of ways, including (https://medlineplus.gov/genetics/understanding/testing/circulatingtumordna/) from tumor cells via exosomes, through secretion of phagocytes post-engulfment of tumor cells, or through cell contents released during apoptosis. Liquid biopsies enable clinicians to identify ctDNA through the presence of somatic variants.

Use in Diagnosis and Treatment

Traditional tumor biomarker testing uses tissue samples from a biopsy or surgical resection. A liquid biopsy involves obtaining body fluid, most commonly peripheral blood, to look for the presence of ctDNA, although it’s also been detected in urine, stool, cerebrospinal fluid, and saliva. Research (https://www.mycancergenome.org/content/page/circulating-dna/) has demonstrated a correlation between the variants found in tumor tissue and ctDNA.

The first commercially available multigene liquid biopsy platform debuted in 2014. The technology has advanced since then and clinical use of liquid biopsy has increased. In 2020, the U.S. Food and Drug Administration approved (https://www.fda.gov/drugs/resources-information-approved-drugs/fda-approves-liquid-biopsy-ngs-companion-diagnostic-test-multiple-cancers-and-biomarkers) two tests that use next-generation sequencing (NGS), which allows (https://www.mycancergenome.org/content/page/circulating-dna/) for evaluation of many different genes at the same time, but only for use in patients with advanced cancer. The combination of liquid biopsy and NGS may help to easily provide comprehensive genetic information about a tumor and assist in selecting treatments effective for the patient’s specific cancer.

Benefits and Barriers

Liquid biopsies have several advantages (https://www.mycancergenome.org/content/page/circulating-dna/) over standard tissue biopsies. Obtaining the sample is quick, minimally invasive, and associated with less pain and fewer complications. It also allows for easier repeat sampling (https://www.cap.org/member-resources/articles/the-liquid-biopsy). Because ctDNA sheds from all tumor sites, the results (https://bmccancer.biomedcentral.com/articles/10.1186/s12885-018-4433-3) may provide a more accurate picture of the patient’s tumor burden and progression compared to a tissue biopsy.

However, the liver and kidneys remove ctDNA from circulation rather rapidly, usually within a few hours. Additionally, ctDNA from solid tumors encompass a very small portion of the total cfDNA. This requires ctDNA detection methods to be highly sensitive (https://www.cap.org/member-resources/articles/the-liquid-biopsy) and specific to provide (https://cjon.ons.org/cjon/24/1/liquid-biopsy-tool-diagnostic-and-prognostic-evaluation-cancers) accurate results and avoid false negatives.

Use in Nursing Practice

• Detection and diagnosis: A standard tissue biopsy is the mainstay for tumor diagnosis; however, liquid biopsy may be an alternative (https://www.healio.com/hematology-oncology/learn-genomics/introduction-to-liquid-biopsies/overview-key-objectives) for patients who are unable to undergo invasive biopsy. Liquid biopsy may also have a future role (https://labtestsonline.org/news/fda-approves-first-liquid-biopsies-test-mutations-multiple-genes) in cancer screening and early detection because of the ease of repeat sampling (https://www.mycancergenome.org/content/page/circulating-dna/).
• Biomarker-specific treatment: Liquid biopsy can detect (https://ascopubs.org/doi/10.1200/PO.18.00397) driver variants to guide treatment selection. Although research (https://www.mycancergenome.org/content/page/circulating-dna/) on using ctDNA to identify predictive biomarkers is emerging, it’s been used for several cancer sites, including lymphoma, breast, thyroid, lung, and colon. 
• Monitoring treatment effectiveness: ctDNA levels may correlate with tumor burden and therefore demonstrate response to treatment. Again, the ease of drawing serial samples enables more frequent, noninvasive monitoring of disease response. Theoretically, serial samples could also detect (https://www.healio.com/hematology-oncology/learn-genomics/introduction-to-liquid-biopsies/overview-key-objectives) molecular changes during therapy and signal drug resistance (https://medlineplus.gov/genetics/understanding/testing/circulatingtumordna/) before radiology evidence (https://www.healio.com/hematology-oncology/learn-genomics/introduction-to-liquid-biopsies/overview-key-objectives).
• Monitoring remission: Lack of ctDNA could indicate (https://medlineplus.gov/genetics/understanding/testing/circulatingtumordna/) that a patient’s cancer has not returned, allowing for easy ongoing monitoring during remission.