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A “blood biopsy” usually means a liquid biopsy: a lab test that looks in blood or another body fluid for cancer cells or material released by a tumor. It can help guide treatment for some people with a known cancer, but it is not a universal cancer test, and a negative result does not always rule out a tumor or a relevant biomarker.
What is a blood biopsy for cancer?
The usual medical term is liquid biopsy. The National Cancer Institute (NCI) defines it as a laboratory test of blood, urine, or another body fluid that looks for cancer cells or molecules released by tumor cells, such as DNA or RNA. One commonly discussed target is circulating tumor DNA (ctDNA): small DNA fragments shed by cancer cells into the bloodstream.
Unlike a tissue biopsy, a liquid biopsy generally starts with a blood draw rather than removing a piece of abnormal tissue. Depending on the test, it may look for genetic changes or other tumor-related signals. Doctors may use results to help find cancer, select or plan treatment, assess response, or check for recurrence. Those are different clinical questions, and a test designed for one purpose should not automatically be assumed to answer another.
How does a liquid biopsy compare with a tissue biopsy?
| Approach | What is sampled | What it can contribute | Important limitation |
|---|---|---|---|
| Liquid biopsy | Blood or another body fluid; the test may look for cancer cells or tumor-related molecules such as ctDNA. | A less invasive way to look for selected tumor signals, including molecular changes that may help guide treatment in some people with known cancer. | The sample may contain too little tumor material to detect a change. A result does not necessarily establish a complete diagnosis or show that a particular treatment will work. |
| Tissue biopsy | A sample of abnormal tissue removed for examination. | Provides tissue for laboratory examination and may be used to assess the tumor and its molecular features. | Obtaining tissue may be difficult or unsafe in some situations; findings can differ from those in a blood sample. |
The NCI notes that blood and tissue findings may not match. A liquid biopsy can be useful when tissue is hard to obtain, but convenience does not make it an automatic substitute. The care team must decide which sample and test fit the clinical question.
When might doctors use a blood-based test?
To help guide treatment for a known cancer
For some people with a diagnosed solid cancer, a clinician may order blood-based genomic profiling to look for changes associated with a targeted therapy or immunotherapy. The result can help inform a treatment decision; it does not by itself prescribe a drug or guarantee that treatment will work. The care team considers the result alongside the cancer type, the person’s overall situation, and other clinical information. A possible treatment may be FDA-approved for that use, used off-label, or available through a clinical trial.
In an October 2020 account, the NCI described FDA approvals of Guardant360 CDx and FoundationOne Liquid CDx for specified genomic profiling uses in people with solid cancers. Those are examples reported at that time, not a complete list or confirmation of current labeling for either test.
To monitor a tumor over time
Because blood can be sampled repeatedly, clinicians may use liquid biopsies at different points in care to look for molecular changes or assess treatment response. Whether serial testing is appropriate depends on the cancer, the test, and the clinical question; the ability to repeat a blood draw does not establish that every test is validated for every monitoring purpose.
To screen people without symptoms
Multi-cancer early detection tests are intended to look for signs of cancer in people who have no symptoms. That is different from profiling a cancer that has already been diagnosed to help choose treatment. The NCI describes multi-cancer tests as an area under study and says important questions remain about potential benefits and harms, including whether this kind of screening reduces cancer deaths.
A positive screening result is not, by itself, a confirmed cancer diagnosis. It may need evaluation with imaging, tissue sampling, or other tests. Screening can also lead to false positives and follow-up procedures, as well as overdiagnosis or overtreatment.
A specific colorectal screening example
In a July 2024 article, the NCI reported FDA approval of Shield as a blood test for primary colorectal cancer screening in people at average risk. That is a specific test, cancer, and population; it does not mean that a blood test screens reliably for every cancer. FDA labeling and screening recommendations can change, so check current guidance for the exact test and intended population.
What does a negative liquid biopsy mean?
A negative result means the test did not detect the particular signal it was designed to find in that sample. It does not necessarily mean that the tumor lacks the alteration or that cancer is absent. Tumors vary in how much DNA they release; the tumor’s location, size, biology, and the assay can all affect whether enough tumor material reaches the blood sample. DNA from normal cells can also make up much of the DNA present.
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For molecular profiling, the NCI’s 2020 discussion describes a non-detect as potentially inconclusive and notes that tissue analysis may be a follow-up when appropriate. The next step depends on the test’s intended use and the individual clinical situation, so the ordering clinician should interpret the result and decide whether further evaluation is needed.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What should you ask before a test?
- What question is this test meant to answer? Ask whether it is for screening, diagnosis, treatment selection, response monitoring, or recurrence assessment.
- What does the test measure? Find out whether it looks for tumor cells, ctDNA, RNA, or another biomarker, and what a detected signal can establish.
- What would a negative result mean? Ask whether a non-detect can be inconclusive and whether tissue testing, imaging, colonoscopy, or another assessment might still be appropriate.
- What follow-up would a positive result require? For screening, clarify how a finding would be confirmed and what procedures might follow.
- Is this use authorized and supported for my situation? Check the test’s current FDA indication and relevant clinical guidance for the cancer type, risk group, and purpose. Marketing for a broader use is not the same as evidence that the use improves health outcomes.
- What are the burdens and potential harms? Discuss false positives and false negatives, downstream testing, possible overdiagnosis or overtreatment, cost, and insurance coverage.
A liquid biopsy result is one piece of clinical information, not a stand-alone verdict. Its meaning depends on why it was ordered, what the assay detects, and how the result fits with other findings.
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