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ctDNA and Minimal Residual Disease: Why the Blood-Test Headlines Need Context
ctDNA tests may detect tumor DNA after treatment. What a result means—and whether it should change care—depends on the cancer and evidence.
Original commentary from the Cancer Explained editorial team.
Please note: this page is educational only — it is not medical advice, and it does not speculate about anyone’s health beyond reliable public reporting. For questions about your own health, talk with your healthcare team.
Two terms that keep getting merged
News stories often use circulating tumor DNA and minimal residual disease as if they were the same thing. They are not. One is a substance in blood. The other is a state of the body. Keeping them apart makes these headlines much easier to read.
This page explains federal sources. It is not medical advice and does not suggest a test or treatment.
What NCI's definitions actually say
Circulating tumor DNA, usually shortened to ctDNA, is described by NCI as small pieces of DNA released into a person's blood by tumor cells as they die. A blood sample can be used to look for it, measure how much is there, and identify specific mutations in it. NCI says ctDNA is being used as a biomarker to help diagnose some cancers, to help plan treatment, and to find out how well treatment is working or whether cancer has come back.
Minimal residual disease, or MRD, is different. NCI defines it as a very small number of cancer cells that remain in the body during or after treatment. It can only be found by highly sensitive laboratory methods, ones able to detect a single cancer cell among a million normal cells.
Then NCI adds a line that rarely survives into news coverage. MRD testing is used mostly for blood cancers such as lymphoma and leukemia. It is also called measurable residual disease.
So when a story applies MRD language to a solid tumor, it is describing an area of active research, not established practice. Our explainers on circulating tumor DNA and minimal residual disease go through each term.
A liquid biopsy is one test among several
NCI describes a liquid biopsy as a test done on a blood sample to look for cancer cells, or pieces of DNA from tumor cells, that are sometimes released into the blood.
It sits alongside other tests, not above them. NCI's guidance on laboratory tests in general is worth carrying into this topic: abnormal results are not a sure sign of cancer, and for many tests it is possible to have normal results even when cancer is present.
How the test is built matters too. Some are designed around mutations found in that person's own tumor. Our page on tumor-informed ctDNA explains what that means and why it changes how a result should be read.
Risk prediction and treatment guidance are different claims
This is the heart of it.
A test can accurately sort people into higher and lower risk of recurrence. That is a prediction. It is not the same as showing that starting, changing, or stopping treatment because of the result leaves people better off.
Both claims can be true. Only one is usually proven. A study that follows people and finds that ctDNA-positive patients relapsed more often has demonstrated the first. It has said nothing yet about the second.
What a strong ctDNA study reports
- The exact test method, fixed in advance.
- The collection schedule. When after surgery or treatment was blood drawn?
- The threshold for calling a result positive, also fixed in advance.
- How often a positive test came before anything appeared on imaging.
- How often recurrence happened anyway after a negative test.
- Whether acting on the result was tested, or only observed.
What a result cannot promise
- A positive result is not the same as a visible recurrence.
- A negative result cannot guarantee that no cancer remains. The test has a floor, below which disease is invisible to it.
- Finding a signal earlier is only useful if acting on it has been shown to help.
- A test validated for one cancer and one time point may not perform the same elsewhere.
Routine follow-up and the scans or tests already recommended should continue unless the care team changes the plan.
Questions about a ctDNA test
- Is this test validated for my cancer and this point in treatment?
- Would either result change what you recommend?
- What are the chances of a false positive or a false negative?
- If it is positive and my scans are clear, what happens next?
How this article was prepared
An AI-assisted editorial system helped prepare this page. No named medical reviewer has reviewed it unless one is listed.
The National Cancer Information Foundation publishes Cancer Explained. This page is for learning. It is not medical advice and does not suggest a test or treatment.
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Put the story in context
Prevention, possible warning signs, screening, and diagnosis
This story relates to Circulating tumor DNA and residual disease. The information below is general: it does not reveal anything else about a public person’s health, and not every point applies to every cancer. Personal advice depends on age, symptoms, family history, exposures, and medical history.
Prevention and risk reduction
Not every cancer can be prevented. Avoiding tobacco, protecting skin from ultraviolet radiation, limiting alcohol, staying active, and receiving recommended HPV or hepatitis B vaccination can lower the risk of certain cancers. A risk factor is not a prediction or a cause in one individual.
Symptoms and possible early signs
Possible signs vary and are often caused by conditions other than cancer. Changes worth discussing include a new lump, unexplained bleeding or weight loss, a persistent cough, lasting bowel or bladder changes, a changing skin spot, or symptoms that persist or worsen. Some early cancers cause no symptoms.
Screening and early detection
Screening looks for certain cancers before symptoms begin. Recommended tests exist only for some cancers and depend on age and risk. Screening can have benefits and harms; it is not the same as evaluating a new symptom, and there is no single routine scan or blood test that reliably screens for every cancer.
How cancer is diagnosed
Diagnosis may involve a history and exam, imaging, laboratory tests, and often a biopsy. Pathology can identify the cancer type and may test biomarkers that guide treatment. Symptoms, screening results, tumor markers, or online stories alone cannot confirm cancer.
A public story may encourage questions, but it should not be used to estimate your risk or choose testing. Contact a healthcare professional about a persistent or concerning change. Seek urgent care for severe or rapidly worsening symptoms.