What Is a 'Liquid Biopsy'? The Blood Test Behind Cancer Headlines

The name is doing a lot of work

A biopsy takes tissue. A liquid biopsy takes blood. The two words together promise something the technology does not yet fully deliver, which is why the phrase turns up in so many headlines.

NCI describes liquid biopsies as tests that look for signs of cancer, or for cancer-related genetic changes, in a sample of blood. Some are already built into cancer care. Others are still being studied as a way to screen people for many cancers at once.

Hold on to that split. It separates what these tests do today from what they are hoped to do later, and most headlines blur the two.

What is actually in the tube

When tumors grow, cells die and break apart, and fragments of their DNA wash into the bloodstream. Those fragments are circulating tumor DNA, or ctDNA. Whole tumor cells can enter the blood as well, though in far smaller numbers.

A liquid biopsy sequences that material. It can report which genetic changes a tumor carries without anyone putting a needle into the tumor itself.

That matters most when a tissue biopsy is hard. NCI points out that a normal biopsy needs surgery or another invasive step. Sometimes a tumor's location, or how unwell a person is, rules one out. A blood draw is safe, cheap, and easy to repeat.

Where it is already standard

The clearest use today is matching a drug to a mutation.

In 2020 the FDA approved two of these tests. They are FoundationOne Liquid CDx and Guardant360 CDx. Both were cleared as companion diagnostics. That means a test that decides whether one specific drug is an option. Both were also cleared for broad tumor profiling.

NCI reported wider approvals later that year. The list grew to include BRCA1 or BRCA2 changes in ovarian cancer, for rucaparib. ALK rearrangements in lung cancer, for alectinib. PIK3CA changes in breast cancer, for alpelisib. And BRCA1, BRCA2, or ATM changes in prostate cancer, for olaparib.

The FDA attached an important caveat, quoted by NCI. If the blood test does not find the change, the tumor tissue should still be analyzed to see whether it is there. A negative blood result is not the end of the question. Our page on liquid biopsy compared with tissue biopsy sets the two side by side.

Where the evidence is still being built

The second use is watching for cancer that has come back before a scan can see it.

NCI covered the BESPOKE CRC study, presented in January 2024. It tested ctDNA in people who had surgery for colorectal cancer. Some still had ctDNA in the blood afterward. In that group, people who got chemotherapy went longer before the cancer came back. In the group with no ctDNA left, there was no sign that chemotherapy helped at all.

That is a useful signal. NCI notes that about 60% of people with this type of colorectal cancer are cured by surgery alone. Treating all of them with chemotherapy means side effects for most, and no gain.

But NCI also quoted Carmen Allegra of its own Division of Cancer Treatment and Diagnosis on the tests already being sold for this: "The problem is, we don't really know how to use them yet." If ctDNA is positive and every scan is clean, nobody yet knows whether to start treatment. Trials are running to answer that. Our page on what tumor-informed ctDNA means explains how those tests are built.

The limit that keeps coming up

Sensitivity. NCI puts it bluntly: liquid biopsies have not been shown to be more accurate than tumor biopsies, and they often fail to detect cancer because there simply is not enough ctDNA in the sample.

How much ctDNA reaches the blood depends on where the tumor is, how big it is, and other biology that is not fully understood. Small, early tumors — exactly the ones a screening test would need to find — shed the least. In 2024, researchers at MIT and the Broad Institute reported a way to raise ctDNA levels in mice for a short time. They called the substances priming agents. NCI's coverage called it very early data that still needs trials in people.

This is why blood tests to screen healthy people are still being studied rather than recommended. That includes multi-cancer early detection tests. Our page on multi-cancer early detection blood tests covers what those trials are asking.

When to get checked

A liquid biopsy is not something to request instead of the checks that are proven to lower cancer deaths. Those are the ones worth booking:

• Colorectal cancer screening from age 45 if you are at average risk
• A mammogram every two years from age 40 to 74
• Cervical screening on the schedule your clinician sets
• A yearly low-dose CT if you are 50 to 80 with a 20 pack-year smoking history and still smoke or quit within 15 years

And no blood test replaces reporting a symptom. A lump, unexplained bleeding, a cough or a change in bowel habit lasting more than three weeks, or weight loss you cannot account for all need a doctor, whatever any test says.

What this does not mean

• "Liquid biopsy" is not one test. Different products look for different things and perform differently. Results have to be read against the specific test used.
• A negative liquid biopsy does not rule out cancer, or rule out a mutation. FDA advises checking tissue when blood comes back negative.
• Detecting ctDNA after surgery is not the same as knowing what to do about it. That is what the current trials are for.
• Approval as a companion diagnostic is not approval for screening. No liquid biopsy is a recommended screening test for people at average risk.

Sources

• NCI Cancer Currents, Cancer "Liquid Biopsy" Blood Test Gets Expanded FDA Approval — https://www.cancer.gov/news-events/cancer-currents-blog/2020/fda-foundation-one-cancer-liquid-biopsy-expanded-approval
• NCI Cancer Currents, ctDNA May Guide Who Needs Chemo After Colorectal Cancer Surgery — https://www.cancer.gov/news-events/cancer-currents-blog/2024/colorectal-cancer-ctdna-may-guide-adjuvant-therapy
• NCI Cancer Currents, Pump Up the Volume: "Priming Agents" May Improve Cancer Liquid Biopsies — https://www.cancer.gov/news-events/cancer-currents-blog/2024/liquid-biopsy-increase-ctdna-in-blood
• NCI, Tumor Markers in Common Use — https://www.cancer.gov/about-cancer/diagnosis-staging/diagnosis/tumor-markers-list
• NCI, What Cancer Screening Tests Check for Cancer? — https://www.cancer.gov/about-cancer/screening/screening-tests

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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