Liquid biopsy for cancer: Hype or help

A blood test that can reveal information about a cancer sounds almost too good to be true. But liquid biopsy is not science fiction. It is already being used in parts of modern oncology — although its capabilities are often exaggerated.

THE SHORT ANSWER

Liquid biopsy looks for cancer-related material circulating in body fluids, most commonly blood.

One of the most important targets is circulating tumor DNA, or ctDNA — tiny fragments of DNA released from cancer cells into the bloodstream.

By analyzing that DNA, doctors may be able to learn something about the molecular characteristics of a tumor without taking another piece of tumor tissue.

That can be extremely useful.

But the usefulness depends on the cancer, the stage of disease, the amount of tumor material circulating in the blood, the test being used and the clinical question being asked.

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WHAT EXACTLY IS A LIQUID BIOPSY?

A traditional biopsy removes cells or tissue directly from a suspicious area or tumor.

A liquid biopsy searches a blood sample — or sometimes another body fluid — for biological material associated with cancer.

Depending on the test, researchers and doctors may examine:

• Circulating tumor DNA (ctDNA)
• Circulating tumor cells
• Other forms of cell-free DNA
• RNA
• Proteins
• Extracellular vesicles
• Other circulating biomarkers

In clinical oncology, ctDNA has become one of the most important areas of liquid-biopsy development.

WHERE LIQUID BIOPSY IS ALREADY USEFUL

This is where the technology moves beyond research hype.

Blood-based genomic tests are already used in selected patients with cancer to identify molecular alterations that can influence treatment decisions.

For example, FDA-authorized liquid-biopsy companion diagnostics can identify particular mutations associated with targeted therapies.

This can be especially valuable when obtaining enough tumor tissue is difficult or when doctors need updated molecular information later in the disease.

Liquid biopsy can also provide a broader picture of tumor genetics because DNA released into the bloodstream can potentially originate from different tumor sites.

WHY REPEATED TESTING MATTERS

Cancer is not biologically static.

As tumors grow and are exposed to treatment, their molecular characteristics can change.

A mutation found when someone was first diagnosed may not tell the entire story months or years later.

Repeated tissue biopsies can sometimes provide new information, but they are invasive and may not always be practical.

A blood sample is much easier to repeat.

This makes liquid biopsy particularly interesting for tracking:

• Changes in tumor mutations
• Response to treatment
• Development of drug resistance
• Changes in tumor burden
• Possible recurrence

WHAT ABOUT MINIMAL RESIDUAL DISEASE?

One of the most exciting areas is the detection of molecular residual disease, or MRD.

After surgery or other treatment intended to eliminate a cancer, imaging may show no visible disease.

But microscopic cancer cells can sometimes remain.

If those cells release detectable tumor DNA into the bloodstream, ctDNA testing may provide evidence that cancer remains at a molecular level.

This has potentially major implications.

It could help doctors distinguish between patients with a higher or lower risk of recurrence and eventually help determine who is most likely to benefit from additional treatment.

WHAT ABOUT COLORECTAL CANCER?

Colorectal cancer is one of the areas where ctDNA research has become particularly important.

In metastatic colorectal cancer, liquid biopsy can help identify molecular alterations relevant to targeted treatment and can reveal mechanisms of resistance that emerge during therapy.

After treatment of localized colon or rectal cancer, ctDNA can also provide powerful information about the risk of recurrence.

This has made molecular residual disease one of the most closely watched areas of colorectal cancer research.

But an important distinction remains:

That second question requires clinical trials.

CAN LIQUID BIOPSY REPLACE A TISSUE BIOPSY?

Sometimes it can provide information that avoids or reduces the need for another tissue sample.

But it should not be thought of as a universal replacement.

Tissue biopsy remains extremely important because it allows pathologists to directly examine the tumor, determine what type of cancer is present and evaluate characteristics that cannot always be reconstructed from circulating DNA.

Blood-based testing also has an important limitation:

Not every tumor releases enough detectable DNA into the bloodstream.

This means a negative liquid biopsy does not always mean that a mutation — or even cancer itself — is absent.

CAN A LIQUID BIOPSY FIND CANCER BEFORE A SCAN?

Potentially — in some circumstances.

ctDNA can sometimes reveal molecular evidence of residual or recurrent disease before it becomes visible on conventional imaging.

That is one reason the technology is so exciting.

But finding something earlier is only clinically useful if doctors understand what the result means and know what action should follow.

An earlier molecular signal does not automatically mean starting treatment immediately will improve survival.

That question must be tested in clinical trials.

CAN ONE BLOOD TEST SCREEN FOR EVERY CANCER?

This is where some of the biggest hype appears.

Researchers are developing multi-cancer early detection tests designed to search for signals from several cancers in people who do not have symptoms.

The concept is scientifically exciting.

But screening healthy populations is very different from using ctDNA in someone already diagnosed with cancer.

A screening test has to answer difficult questions:

• How many cancers does it actually detect?
• How many early cancers does it miss?
• How many false alarms does it produce?
• Can it identify where the cancer originated?
• What tests should follow a positive result?
• Most importantly: does screening reduce cancer deaths?

Those questions cannot be answered simply by showing that cancer DNA can sometimes be detected in blood.

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WHAT THE EVIDENCE SAYS

Liquid biopsy is best understood as a collection of technologies rather than one single test.

The evidence is strongest for particular applications, including molecular profiling and selected companion-diagnostic uses.

Evidence is also rapidly developing for treatment monitoring, detection of resistance and molecular residual disease.

Other applications — especially broad population screening and using every positive MRD result to determine treatment — require more evidence.

That distinction matters.

WHAT’S OVERHYPED?

Statements such as these go too far:

• “A blood test can now detect every cancer.”
• “Liquid biopsy has replaced tissue biopsy.”
• “A negative ctDNA result means there is definitely no cancer.”
• “A positive ctDNA result always means treatment should start immediately.”
• “Liquid biopsy can tell exactly whether cancer will return.”

Liquid biopsy is powerful precisely because it can answer certain questions extremely well.

That does not mean it can answer every question.

WHAT THIS MEANS FOR PATIENTS

If you already have cancer, the useful question is not simply:

“Should I get a liquid biopsy?”

A better question is:

Depending on the situation, that question might involve:

• Finding a targetable mutation
• Understanding resistance to a treatment
• Looking for molecular residual disease
• Monitoring disease over time
• Determining whether additional tissue testing is needed

QUESTIONS FOR YOUR DOCTOR

SOURCES

U.S. Food & Drug Administration
FDA-Authorized Companion Diagnostic Devices
View source

U.S. Food & Drug Administration
FoundationOne Liquid CDx
View source

Nature Reviews Clinical Oncology — 2026
Towards liquid biopsy-based analysis of antitumour immunity
View review

Nature Reviews Clinical Oncology — 2026
Evolving roles of liquid biopsy in precision medicine for colorectal cancer
View review

Nature Reviews Genetics — 2026
Advancing the applications of liquid biopsies in oncology
View review

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LAST REVIEWED
September 3, 2026