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Circulating tumor DNA (ctDNA)


Circulating tumor DNA, or ctDNA, tests analyze a blood sample to look for fragments of genetic information (DNA) shed by cancer cells into the bloodstream. Cancer cells have DNA mutations that normal cells do not. Another term used for this testing is liquid biopsy—a reference to the use of a blood sample for this test rather than a tissue sample, as in a traditional biopsy.


In metastatic breast cancer, ctDNA testing is often used to look for specific genetic mutations that can help doctors choose among targeted therapies. These mutations, or gene changes, develop over time; they are different from inherited mutations that run in families, such as BRCA1 and BRCA2. For example, testing for mutations in genes such as PIK3CA and ESR1 can determine eligibility for treatments such as alpelisib (Piqray) and elacestrant (Orserdu), respectively.

Today ctDNA tests are also being used to detect the presence of minimal residual disease, also called microscopic residual disease or MRD. These are tiny amounts of cancer that are much too small to show up on imaging tests, such as MRI or CT scans. These MRD ctDNA tests analyze the blood for multiple genetic mutations associated with a specific type of cancer, which are shed as small DNA fragments by any microscopic cells that are still present. In early-stage and metastatic cancer, the results could be used to assess whether a cancer is responding to treatment. In early-stage cancer, the results can help predict the likelihood of later recurrence after treatment. Knowing that there are microscopic cancer cells in the body indicates a higher risk of the cancer coming back.

CtDNA tests for minimal residual disease are still being studied. They have shown promise in colorectal, lung, urothelial (urinary system), and breast cancers. They are not yet considered part of standard care for breast cancer, but that could change as more information becomes available through clinical trials. One MRD ctDNA test, Signatera, is covered by Medicare for monitoring people with stage IIB or higher breast cancer that is hormone receptor-positive, HER2-positive, or triple-negative. So it’s important to be aware of these tests, where the research currently stands, and their potential advantages and disadvantages.


Types of ctDNA tests for minimal residual disease

MRD ctDNA tests analyze the plasma, or liquid portion of the blood separated from blood cells, for fragments of mutated DNA shed by cancer cells. The tests typically would be done every few months to track whether ctDNA is present. There are two main types of tests:

  • Personalized tests (for example, Signatera and Invitae): For these ctDNA tests, a sample of the tumor tissue is analyzed first to pinpoint specific DNA mutations that are driving the cancer. This is done by comparing the cancer cells to normal, healthy cells. These mutations are then used as the basis for creating the blood test. The number of mutations included can vary from 16 all the way to 50, depending on the test manufacturer. The initial tissue analysis and test creation can take up to one month. After your blood is drawn, the results usually come back in 1 to 2 weeks.
  • Tumor-naïve tests (for example, Guardant Reveal): These tests are not personalized to the tumor, so they don’t require a tissue sample. Instead, they test the blood for DNA fragments that are “hotspot” mutations commonly associated with a specific type of cancer. Results usually come back in 1 to 2 weeks. One challenge of these tests is telling the difference between mutations that are related to cancer and those that are simply the result of aging.

What the results of MRD ctDNA tests mean

A positive result means that ctDNA is present in the blood and there is microscopic disease in the body. A negative result means that there is no evidence of microscopic disease. Personalized tests, such as Signatera, can also indicate the level of ctDNA that was found.

Studies have shown that a positive MRD ctDNA test result accurately predicts eventual recurrence in colorectal cancer, non-small cell lung cancer, breast cancer, and urothelial cancer. A negative result suggests that the risk of recurrence is low. The key question that remains to be answered in clinical trials is how to use the results to improve on the current standard of care.


Potential uses for MRD ctDNA tests

Researchers are interested in MRD ctDNA tests because the tools doctors now have for evaluating response to cancer treatment and monitoring for recurrence aren’t perfect. Existing tools include physical exams, imaging scans, and testing CEA protein levels—as well as using the features of the cancer itself to gauge recurrence risk. However, it’s still difficult to predict which cancers are more likely to recur and could benefit from more intensive treatment. In breast cancer, for example, up to 30 percent of people with early-stage cancer will eventually have a recurrence after apparently successful treatment. Knowing whether microscopic disease is present could provide information that existing tests can’t.

Clinical trials are investigating whether or not making treatment decisions based on MRD ctDNA test results is better than using the tools now available as standard of care.

Potential uses for early-stage cancer include MRD ctDNA testing at various times, such as:

  • After neoadjuvant therapy (treatment given before surgery), to evaluate whether any cancer is still present
  • After surgery, to inform decisions about the treatment plan moving forward: asking questions such as, is this cancer high-risk or low-risk, and does it call for more intensive or less intensive treatment?
  • During post-surgical treatments, such as chemotherapy or targeted therapies, to assess how well the cancer is responding
  • After treatment is finished, to monitor for recurrence

In cases of metastatic cancer, most people will have a positive MRD ctDNA test, since cancer cells are known to be present in one or more areas of the body. However, changing MRD ctDNA levels measured by a test such as Signatera could help assess how well the cancer is responding to therapy. If ctDNA levels are falling, or they become undetectable in the blood, this would suggest that a treatment is working.

Clinical trials for MRD ctDNA testing with Signatera in colorectal cancer are furthest along, showing the usefulness of the test in guiding treatment planning. Many doctors now use this test in people with stage II or stage III colorectal cancer to determine whether chemotherapy is needed after surgery, to monitor for recurrence after treatment, or both. In colorectal cancer, MRD ctDNA testing is typically used in combination with standard tools such as CEA tests and imaging scans.

With more clinical trials for other cancer types, MRD ctDNA testing may come into wider use.


MRD ctDNA testing in breast cancer

CtDNA testing for minimal residual disease is not yet a standard part of care for breast cancer, according to guidelines issued by organizations such as the National Comprehensive Cancer Network and the American Society of Clinical Oncology. However, some research suggests that it could be useful for predicting recurrence and assessing response to therapy in people with breast cancer.

For example, the Exploratory Breast Lead Interval Study (EBLIS) used Signatera to monitor people with all types of breast cancer every six months after surgery. The study found that a positive test result predicted eventual recurrence as much as 1 to 2 years before any cancer showed up on imaging scans (median lead time: 8.9 months).

Another trial that used Signatera, called I-SPY2, found that a negative ctDNA result after the first neoadjuvant treatment (therapy given before surgery) was a good indicator that the cancer would respond completely to treatment—also called pathologic complete response, or PCR. In addition, among those who still had evidence of disease but a negative ctDNA result after a few treatments, most (93%) remained free of distant recurrence. Those who tested positive for ctDNA had a higher risk of metastatic recurrence later on.

Ongoing and future clinical trials need to answer some key questions about MRD ctDNA testing in breast cancer, such as:

  • What is the risk of a false-positive result, meaning that the test comes back positive but microscopic residual disease is not really present? What about false negatives (negative result but MRD really is present)?
  • Are ctDNA test results always a reliable predictor of response to treatment and recurrence risk?
  • Do people who are on a ctDNA-guided treatment plan do better than those who follow the current standard of care and are monitored with imaging scans, physical exams, and CEA blood level testing?
  • If a person is being monitored with ctDNA testing and the result comes back positive, what is the best course of action? More frequent monitoring? More therapy? Watch and wait?
  • What is the best type and timing of therapy to use after a positive ctDNA test result? Does starting treatment sooner lead to better outcomes than waiting until cancer is visible on imaging scans?
  • How long should MRD ctDNA testing continue? If a person treated for early-stage breast cancer remains cancer-free for a certain number of years, is testing still warranted?

Until these questions are answered, people and their doctors need to work together to determine whether Signatera or another MRD ctDNA test makes sense on a case-by-case basis.


Talking with your doctor about MRD ctDNA testing

Although MRD ctDNA testing is not part of current evidence-based guidelines, some doctors may decide to use a test in combination with other tools, such as the features of the cancer in the pathology report, imaging scans, and CEA blood testing. Medicare has approved coverage of the SIgnatera test to monitor response to treatment and recurrence in people with stage IIb or higher breast cancer.

If you and your doctor are considering using Signatera or another MRD ctDNA test, it’s important to have an in-depth discussion about potential pros and cons. Keep in mind that a negative MRD ctDNA result on its own should never be used as a reason to forgo treatments such as chemotherapy or hormonal therapy, or stop them earlier than recommended. These tests are only a few years old; the evidence behind current treatment recommendations was developed over decades.

Your doctor can help you understand the value of ctDNA testing in light of your individual situation, the features of your cancer, and your risk of recurrence. Think about how you might react to any results you receive, and whether they would have a positive or negative impact on your day-to-day life.

You also can talk to your doctor about enrolling in a clinical trial that is studying MRD ctDNA tests in people with breast cancer. Ask your doctor if they know of any trials in your area. You also can search clinical trial databases using the term “ctDNA.”

Here are some of the potential pros and cons of testing:


For some people, MRD ctDNA testing may provide peace of mind and reduce uncertainty if they’re concerned about recurrence. Potential advantages include:

  • Having another piece of information to judge whether there is any microscopic cancer left behind after surgery or other treatments
  • More accurate information about whether an early-stage cancer that appeared to be treated successfully is likely to recur later on
  • A clearer sense of whether metastatic disease is responding to treatment


At the same time, there are some disadvantages to testing.

  • It’s not yet known whether Signatera and other ctDNA tests are 100% accurate. A negative result may give a false sense of security, while a positive result may lead to high levels of anxiety.
  • A negative result does not mean the cancer definitely will not come back. While a positive result does appear to predict the likelihood of a future recurrence, it also does not mean the cancer will return in 100% of those with such a result.
  • There is uncertainty about how to act on a positive MRD ctDNA result. Doctors don’t yet know whether more frequent imaging, more treatment, or changes in treatment will make a difference in outcomes. They also don’t know if there is a benefit to starting or restarting treatment before cancer shows up on imaging scans.
  • A positive result could lead to overtreatment, with people experiencing difficult side effects from treatments that weren’t necessary. Similarly, a negative result might tempt someone to discontinue a treatment with bothersome side effects (such as hormonal therapy) too early—which could increase the risk of recurrence.
  • Because these tests are not FDA-approved, they might not be covered by health insurance. Signatera is furthest along in development and may be covered in light of Medicare’s approval for its use in stage IIb and higher breast cancer. Some companies also offer financial assistance for tests that aren’t covered.

Until more information is available from clinical trials, your best course of action is to speak with your doctor about your individual preferences and needs.

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Reviewed and updated: March 30, 2023

Reviewed by: Sameer Gupta, MD, MPH , Reshma L. Mahtani, DO


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