Liquid Biopsy: Blood Testing for Tumor Cells and Tumor DNA

Nov 7

4 min read

Updated: 3 days ago

Written and medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: November 2026 · How we review

Quick Answer

A liquid biopsy is a blood test that looks for signs of cancer shed by a tumor into the bloodstream. It's mainly used in advanced cancer to track response to treatment and find gene changes that point to targeted therapies. It isn't reliable enough to diagnose cancer on its own; a tissue biopsy remains the gold standard.

What the research says

Tumor cell counts predict outlook: in 177 people with metastatic breast cancer, those with 5 or more circulating tumor cells per 7.5 mL of blood before starting a new treatment had much shorter progression-free survival (2.7 vs 7.0 months) and overall survival than those with fewer (Cristofanilli M et al., N Engl J Med, 2004) [2]
Early detection is still limited: in a validation study of 4,077 people, a multi-cancer blood test had 99.5% specificity but detected only 51.5% of cancers overall, including just 16.8% of stage I cancers compared with 90.1% of stage IV cancers (Klein EA et al., Ann Oncol, 2021) [3]

Key takeaways

A blood test that looks for tumor cells or tumor DNA.
Mainly used for advanced cancer to predict outlook and guide treatment.
Can find gene changes that match targeted therapies.
Less invasive and easy to repeat.
Doesn't replace a tissue biopsy for diagnosing cancer.

What does a liquid biopsy look for?

Comparison of circulating tumor cells and circulating tumor DNA found by liquid biopsy

Liquid biopsies detect pieces of tumors in the blood.

Circulating tumor cells (CTCs): whole cancer cells that have broken away from a tumor and travel in the blood.
Circulating tumor DNA (ctDNA): fragments of DNA released by tumor cells, which can reveal genetic changes that guide treatment. Sources: [1]

Why is it done?

List of liquid biopsy uses including predicting outlook in metastatic cancer and finding gene changes for targeted therapy

Liquid biopsy is used mainly in advanced cancer.

Prognosis: fewer CTCs are linked with a better outlook than many CTCs, and repeat testing can track the disease.
Treatment choice: ctDNA tests can show gene changes that make a targeted therapy more likely to work, such as EGFR changes in non-small cell lung cancer.
When a tissue biopsy isn't safe: for example, if you're too unwell or the tumor is hard to reach. Sources: [1, 2]

It's most often ordered for metastatic cancer when current treatment isn't working, including metastatic breast cancer, prostate cancer and colon cancer. Sources: [1]

FDA-approved liquid biopsy tests

Test	What it detects	Main use
CellSearch CTC Test	Circulating tumor cells	Predicting outlook in metastatic breast, prostate or colon cancer
cobas EGFR Mutation Test v2	ctDNA with EGFR changes	Non-small cell lung cancer
Guardant360 CDx	ctDNA gene changes	Choosing treatment
FoundationOne Liquid CDx	ctDNA gene changes across many cancers	Choosing treatment

Sources: [1]

How is it done?

A healthcare provider draws blood from a vein, which feels like any routine blood test. In the lab, a machine separates the blood cells from the plasma, and a pathologist looks for CTCs or ctDNA. Sources: [1]

Liquid biopsy vs tissue biopsy

Comparison of liquid biopsy and tissue biopsy

A tissue biopsy is still the gold standard for diagnosis.

A tissue biopsy removes part of the tumor and is the gold standard for diagnosing cancer. A liquid biopsy is less invasive and less painful and can be repeated, but a single blood sample may not show signs of a tumor even when one is present. Sources: [1, 3]

Understanding your results

Results usually take two to three weeks. They show whether CTCs or ctDNA were found. A positive result can help identify the cancer type and genetic changes. A negative result doesn't rule out cancer. Research into using liquid biopsy for early cancer screening is ongoing. Sources: [1, 3]

Conclusion

A liquid biopsy offers a simple way to learn about advanced cancer through a blood draw, helping predict outlook, track treatment and find targeted therapy options. It complements, rather than replaces, imaging and tissue biopsy.

Keep learning: read about blood tests for cancer and tumor markers.

Frequently asked questions

What is a liquid biopsy?

A blood test that looks for tumor cells or tumor DNA.

What does it detect?

Circulating tumor cells and circulating tumor DNA.

Can a liquid biopsy diagnose cancer?

Not reliably on its own; a tissue biopsy is needed.

Who gets a liquid biopsy?

Mostly people with advanced cancer.

Does it hurt?

It feels like a routine blood draw.

How long do results take?

Usually two to three weeks.

Can it choose my treatment?

It can find gene changes that point to targeted therapies.

Can a liquid biopsy miss cancer?

Yes. A tumor's signs may not appear in one sample.

Is it used for screening?

Research is ongoing; it isn't a standard screening test yet.

Is it the same as a tumor marker test?

No. Tumor markers measure proteins; liquid biopsy looks for tumor cells or DNA.

References

Cleveland Clinic. Liquid Biopsy: What It Is & Procedure Details. Cleveland Clinic. Last updated August 11, 2022. https://my.clevelandclinic.org/health/diagnostics/23992-liquid-biopsy
Cristofanilli M, Budd GT, Ellis MJ, et al. Circulating tumor cells, disease progression, and survival in metastatic breast cancer. N Engl J Med. 2004;351(8):781-791. PMID 15317891. https://doi.org/10.1056/NEJMoa040766
Klein EA, Richards D, Cohn A, et al. Clinical validation of a targeted methylation-based multi-cancer early detection test using an independent validation set. Ann Oncol. 2021;32(9):1167-1177. PMID 34176681. https://doi.org/10.1016/j.annonc.2021.05.806

Medical disclaimer

This article is for general education only and does not replace advice from a qualified healthcare professional. Talk to your doctor about your own diagnosis and treatment, and seek emergency care right away for severe or sudden symptoms.