July 2023 | George M. Pikler, M.D., Ph.D., FACP

Liquid Biopsies for Cancer: What to Know and What to Expect

A liquid biopsy is a test that can isolate circulating extracellular nucleic acids (cell-free DNA; cfDNA), circulating tumor DNA (ctDNA), and circulating tumor cells (CTCs) from the blood. cfDNA is DNA freely circulating in the blood, which may or may not be of tumor origin, whereas ctDNA is of tumor origin. Circulating tumor cells (CTCs) are cancer cells that are shed into the blood. They usually only last for 1–2.5 h in the circulation prior to their destruction by the immune system, but a small fraction can survive and seed distant metastatic sites.

In contrast to core needle biopsies and fine needle aspiration biopsies which involve a surgical removal of a small amount of tissue from a tumor or suspicious area, and requires a pathologist to examine the tissue under a microscope for the presence of cancer cells, a liquid biopsy, is performed using a simple blood draw. The blood sample is then sent to a laboratory for analysis. ctDNA/cfDNA and CTCs can be assayed using next-generation sequencing (NGS). The results may be available within two to three weeks.

Several liquid biopsy tests are under development and in various stages of research. Four have been approved by the U.S. Food and Drug Administration (FDA). One analyzes a blood sample for the presence of circulating tumor cells and is used to predict the likely outcome for people with metastatic breast, prostate, or colon cancer. Other tests detect mutations, or common genetic errors, in the DNA from tumor cells circulating in the blood. By identifying these mutations, we can determine whether a type of treatment called targeted therapy may be helpful in treating the cancer. Targeted therapies work by targeting the specific mutations contributing to the cancer’s growth and survival.

Clinical applications:>br> • To detect cancer in its earliest stages and those that may not yet have been found with routine screening tools.
• In addition to a tumor biopsy to support an initial cancer diagnosis or if getting a tissue sample through a needle biopsy is not possible or if results are needed more quickly.
• To monitor how well tumors respond to a specific treatment before these changes can be seen on imaging tests, such as on CT and MRI scans.
• To see if they can help detect cancer recurrence in people who have previously received cancer treatment.

Limitations of liquid biopsies:
• ctDNA/cfDNA can be shed in only small amounts and not all patients will have detectable levels, especially those with low tumor burden. Because of the small amount of material shed in the circulation, sequencing can be difficult and expensive.
• Not all detectable cfDNA alterations are cancer related. A standard biopsy may be needed to confirm a cancer diagnosis.
• Not all ctDNA/cfDNA is equally shed from the primary tumor and metastases, so it is unclear if the alterations detected accurately represent tumor heterogeneity.
• Shedding of ctDNA can be suppressed by treatment and may be limited at certain disease sites.
• Standardization across laboratories and vendors is needed to ensure reproducibility.

J Hematol Oncol. 2022; 15: 13. Sep 12

Erica Cross, PA


Erica is a board certified Physician Assistant. She obtained her Master’s degree in Physician Assistant studies from Our Lady of the Lake College in Baton Rouge, LA. She began practicing in 2011 and has worked clinically in Orthopedics and Dermatology. The majority of her career has been spent in a Dermatology practice where she assisted in Mohs surgery, treating various types of skin cancer. She also teaches in the medical simulation department at the University of South Alabama and enjoys every aspect of medical education.