Circulating tumor cell (CTC) tests

Circulating tumor cells (CTC) tests look for and count whole cancer cells that have broken away from a tumor and entered the bloodstream.

CTCs can travel in the blood to other organs or tissues, where they may form a new tumor. Knowing the number of CTCs in the blood can help doctors understand whether cancer may be growing and/or spreading. CTC testing can also monitor how well cancer is responding to treatment.

CTCs are biomarkers, or biological markers: signs of disease or changes in the body that can be tested in blood or tissue.

How CTC tests work

CTC tests are a type of liquid biopsy. Liquid biopsies look for cancer cells in a blood sample, rather than in a tumor tissue sample.

For a CTC test, you’ll go to a lab to have a blood sample taken by needle from a vein in your arm. The vein is usually on the inside of your elbow, but sometimes a vein on the wrist or back of the hand is used.

Healthcare providers can also take a blood sample through a chemotherapy port if you have one. This is a small device placed under the chest skin near a vein so chemotherapy can be delivered.

Your healthcare provider will send your blood sample to a lab that will count the number of CTCs in the sample.

CTC tests measure the number of tumor cells in your blood and can help predict:

• Whether cancer is growing and/or spreading
• How well a treatment is working to control the cancer

CTC tests are different than ctDNA tests

CTC tests are not the same as ctDNA tests:

• CTC tests can look for whole cancer cells that broke away from a tumor to predict whether cancer may be growing or spreading.
• ctDNA tests can:
  • Check the blood for small pieces of DNA that are released by dying cancer cells to confirm gene mutations and other biomarkers; this can help guide treatment decisions
  • Be used to test for biomarkers in the blood if a tumor is located in an area that is difficult to biopsy for a tissue sample
  • Detect minimal residual disease—tiny amounts of cancer that are too small to show up on imaging tests such as a CT or MRI scan
  • Predict the likelihood of early-stage cancer coming back

Who gets CTC tests?

Right now, CTC tests are only available to people with metastatic breast, prostate, or colon cancer.

CTC tests may also be used if you participate in a clinical trial. People with early-stage breast cancer may be able to get a CTC test as part of a clinical trial.

What CTC test results mean

Only one CTC test, the CellSearch Circulating Tumor Cell Test, is FDA-cleared to use in metastatic breast cancer outside of research.

CellSearch CTC test results are reported as the number of CTCs in 7.5 milliliters of blood (about 1.5 teaspoons). Repeating the test can show if the CTC count is increasing or decreasing and suggest if the cancer is responding to treatment.

To see how the cancer is responding, doctors will look for whether the CTC count is:

• Below five or decreasing
• Above five or increasing

If your doctor recommends CTC testing for you, they will also recommend how often to have the test (such as every 3 months).

To learn about other tests used in breast cancer, visit the Testing and precision medicine page.

Uses and limits of CTC testing

CTC tests are currently used to count whole cancer cells in the blood. These tests are still in clinical trials to research other uses, such as looking for biomarkers that can help you and your doctors make treatment decisions.

How CTC tests are being used now

CTC tests are used to count the number of whole metastatic breast cancer cells in the blood. Knowing these numbers can help doctors:

• Assess how well treatment is working
• Predict whether the cancer is likely to grow and spread
• Predict whether the cancer is likely to respond to treatment

CTC tests are still fairly new in breast cancer care. For this reason, these tests are used in combination with other tests, such as scans, to diagnose and monitor metastatic breast cancer. Researchers are learning more about how these tests can be used as more people use them and as more clinical trials are done.

It is important to know that at this time, clinical trials have not shown that CTC testing can be used to guide decisions about changing treatment. Future clinical trials may provide more information on how CTC testing can be used in breast cancer care.

If you have been diagnosed with metastatic breast cancer and your doctor has recommended CTC testing in addition to scans, ask why. Knowing the goal of any test or treatment can help you and your doctor work together to make informed decisions about your care.

How CTC tests are being studied in clinical trials

In clinical trials, researchers are studying CTC testing to:

• Learn more about the test’s ability to detect biomarkers such as estrogen receptors, gene mutations, and PD-L1 in metastatic breast cancer. Biomarkers can help guide treatment decisions.
• Learn more about its potential to tell if CTC biomarkers are different than biomarkers in a tumor.
• Learn more about how CTC testing for biomarkers may be helpful when it is not possible to biopsy a tumor. Researchers are looking at whether CTC testing for biomarkers in the blood can be helpful if a metastatic breast cancer tumor is in an area that is difficult to biopsy.
• Understand whether it is useful to test for CTCs in early-stage breast cancer.
• Learn about CTC tests’ potential to monitor for metastatic recurrence after early-stage treatment is complete.

These trials may provide more information on how CTC tests may be used to:

• Monitor the cancer
• Help you and your doctor choose treatments that can treat and control the cancer while minimizing side effects

To learn about participating in breast cancer research involving CTC tests, other biomarker tests, new treatments, and more, visit Clinical trials. You can also visit:

• The Metastatic trial search tool
• The Early-stage breast cancer trial search tool

Paying for CTC tests

CellSearch is currently the only CTC test that is FDA-cleared for use in metastatic breast cancer.

• Medicare, private insurance, and a financial assistance program can help you pay for CellSearch. Learn more about financial assistance.
• Talk with your healthcare team and your insurance company about coverage for CTC tests.
• You can also ask about the possibility of joining a clinical trial. Some trials may provide CTC tests at no cost.

Visit the financial matters page for more information and resources on paying for care.