Liquid biopsy FAQ

This resource focuses on the use of liquid biopsy testing to assess for targeted treatment options in solid tumors.

What is a liquid biopsy?

Liquid biopsy testing is performed to assess or detect cancer. It is used for identifying targeted treatment options and typically assess many genes and, in some cases, other types of biomarkers. There are targeted testing options for specific clinical needs, such as identifying minimal residual disease in specific cancer types and monitoring treatment response.

Liquid biopsy testing involves the collection and analysis of biomarkers from cancer DNA present in the peripheral blood. No solid tumor sample is necessary.

Most often, liquid biopsy tests analyze circulating tumor DNA (ctDNA), which are pieces of DNA that are sloughed off cancer cells. Liquid biopsy may also be called cell free DNA (cfDNA) testing.

When should a liquid biopsy test be ordered?

Liquid biopsy testing is offered to assess for targeted treatment options, similar to biomarker testing that is performed on a tumor sample. Liquid biopsy testing is most often ordered in the setting of advanced and/or recurrent cancer. Indications for liquid biopsy include the following:

  • at the time of initial biomarker testing, if there is not sufficient tissue available for tissue-based testing;
  • when there are multiple metastases to better assess DNA across disease sites; and
  • at the time or recurrence or disease progression to assess for new variants to target or the development of resistance variants.

What types of biomarkers does a liquid biopsy test detect?

DNA variants. Liquid biopsy tests can detect small and moderate sized variants in DNA including changes to single DNA bases (also known as single nucleotide variants or SNVs), copy number variants (CNVs), small insertions and deletions (indels) and some fusions.

Certain immunotherapy markers. Most liquid biopsy tests can evaluate tumor mutation burden (TMB) and microsatellite instability (MSI). Detection of PD-L1 requires a protein assay which is not included in most liquid biopsy tests.

Somatic and germline variants. Cancer cells contain a mix of variants that are present in every cell in the body (germline) and those that have developed in certain cells over time (somatic). Both types of variants are present in the DNA from cancer cells. Like some types of tumor-based tests, liquid biopsy testing is not optimized to definitively determine the source of the variant – germline or somatic. Additional, dedicated germline testing may be needed to determine if a variant is present in the germline, which could impact treatment options, future cancer risk, and relatives’ cancer risk.

Non-cancer associated genomic variants. Genomic changes are not unique to cancer cells. Non-cancer cells, specifically those in the blood or bone marrow, can develop genomic variants in cancer-associated genes in older individuals or those who have undergone cancer treatment. This phenomenon is called clonal hematopoiesis of indeterminate potential (CHIP). Individuals with CHIP do not have detectable hematological cancer but may have an increased risk to develop a malignancy and may be at increased risk for cardiovascular disease. Variants due to CHIP should not be targeted for treatment as they are unrelated to the active cancer. It can be challenging to differentiate variants due to CHIP and those due to the active cancer process.

What can liquid biopsy tests NOT detect?

The ability of a liquid biopsy test to detect biomarkers is dependent on having enough DNA being sloughed from cancer cells. When there is not enough DNA to assess, it is referred to as low tumor shedding. There are several things that influence the amount of DNA available in the blood including the following:

  • Cancer type. Some types of cancer release less DNA into the blood, including brain, renal and thyroid cancers. DNA shedding is typically higher in colorectal, lung, and breast cancers.
  • Disease progression. Generally, cancer cells shed more DNA when they are replicating, and the cancer is growing. Cancers that are being successfully treated are less likely to shed sufficient DNA for analysis.

Non-DNA biomarkers. Liquid biopsy tests rely on ctDNA and cannot detect non-DNA biomarkers. While some fusions can be detected through DNA, more comprehensive assessment for the presence of fusions requires RNA analysis. Protein-based biomarkers such as PD-L1 cannot be assessed.

Large DNA deletions and duplications. Like many tumor-based tests, the techniques used in liquid biopsy testing are not optimized to detect large DNA-based changes, such as multiple exon or whole gene deletions.

What are benefits of using liquid biopsy tests?

A primary benefit of liquid biopsy testing is its non-invasive nature and ability to collect cancer DNA when a tumor is not accessible or there is insufficient sample to analyze. Additional benefits include:

  • High sensitivity
  • Easy sample collection; allows for testing when a solid tumor biopsy is challenging
  • Fast turn-around time (~7-10 days)
  • Detection of variants present in cancer cells throughout the body as opposed to variants present in a single tumor location
  • Low failure rate in the presence of active tumors

What are limitations of using liquid biopsy tests?

In many cases, assessing a tissue-based tumor sample is consider the gold standard. Some limitations of liquid biopsy testing include:

  • May not identify all types of biomarkers present in the tumor (e.g., fusions)
  • Increased risk for false negative due to low concentrations of ctDNA in the blood. This means that a variant might be present but not detected.
  • May identify fewer potential targets compared to tumor-based biomarker test
  • Cannot differentiate definitively the source of the variants, making it challenging to identify which are associated with the active cancer as opposed to CHIP (variants in non-cancer cells), an incidental germline variant, or a potential second malignancy can be challenging.

Learn more

Explore Cancer Biomarker Testing (CME|CNE). Learn about benefits, limitations, and challenges of using large biomarker tests.

Choosing the Best Genomic Tumor Test (CME|CNE). Learn about the benefits and limitations of different genomic tumor test options for patients with cancer and how to determine the best test for each patient.

Hereditary Cancer Syndromes: Are Your Patients at Risk? (CME|CNE). Develop skills and knowledge to identify those patients most appropriate for genetic testing, choosing the right test, and using the results to develop a management plan.

Interpreting Cancer Biomarker Testing – When is Additional Testing Needed? (CME|CNE). Learn when additional cancer biomarker testing is indicated for further evaluation of genome-informed therapy. 

Testing to Identify Genomic Variants Related to Cancer. Defines and compares benefits and limitations of types of genomic testing for identifying targeted treatments.


Chan, H. T., Chin, Y. M., & Low, S. K. (2022). Circulating Tumor DNA-Based Genomic Profiling Assays in Adult Solid Tumors for Precision Oncology: Recent Advancements and Future Challenges . Cancers (Basel),14(13).

De Mattos-Arruda, L., & Siravegna, G. (2021). How to use liquid biopsies to treat patients with cancer . ESMO Open, 6(2), 100060.

Heitzer, E., Haque, I. S., Roberts, C. E. S., & Speicher, M. R. (2019). Current and future perspectives of liquid biopsies in genomics-driven oncology . Nat Rev Genet,20(2), 71-88.

Nikanjam, M., Kato, S., & Kurzrock, R. (2022). Liquid biopsy: current technology and clinical applications . J Hematol Oncol,15(1), 131.

Olson, D. R. (2018). The “Liquid” Biopsy . College of American Pathologists.

Reed, E. K., Steinmark, L., Seibert, D. C., & Edelman, E. (2019). Somatic Testing: Implications for Targeted Treatment . Semin Oncol Nurs,35(1), 22-33.


This resource was developed as part of the Maine Cancer Genomics Initiative (MCGI) and is supported by The Harold Alfond Foundation, Maine Cancer Foundation and The Jackson Laboratory.

Updated May 2023