How Liquid Biopsies Detect Cancer From a Blood Draw

A Blood Test That Finds Cancer

For decades, detecting cancer meant cutting into tissue. Surgeons removed samples with needles or scalpels, sent them to pathologists, and waited days or weeks for results. Now a simpler approach is reshaping oncology: the liquid biopsy, a standard blood draw that can reveal whether a tumor is present, what mutations drive it, and whether treatment is working.

The technology hinges on a biological fact discovered in the 1940s but only recently harnessed. As cancer cells grow, divide, and die, they shed tiny fragments of their DNA into the bloodstream. These fragments — called circulating tumor DNA (ctDNA) — carry the same genetic mutations as the original tumor. By fishing them out of a blood sample, scientists can read the molecular fingerprint of a cancer without ever touching the tumor itself.

How It Works

A liquid biopsy begins like any routine blood draw. A clinician collects a small sample — typically two to four tubes — and ships it to a specialized laboratory. There, technicians separate the plasma (the liquid portion of blood) from blood cells using a centrifuge.

The plasma contains a mix of cell-free DNA shed by all cells in the body. The challenge is isolating the cancer-derived fragments, which may represent less than 0.1% of the total. Laboratories use next-generation sequencing (NGS) or polymerase chain reaction (PCR) techniques to amplify and read these fragments, scanning for known cancer-associated mutations across dozens or even hundreds of genes.

According to Foundation Medicine, its FoundationOne Liquid CDx test analyzes over 300 genes from a single blood draw, making it the most comprehensive FDA-approved liquid biopsy available. Results typically arrive within one to two weeks — roughly two to three times faster than traditional tissue biopsies, according to the Cleveland Clinic.

What Liquid Biopsies Can Do

Clinicians currently use liquid biopsies in three main ways:

• Guiding treatment: By identifying specific genetic mutations, doctors can match patients with targeted therapies. The FDA-approved Guardant360 CDx test, for example, profiles genomic alterations across all solid tumors and serves as a companion diagnostic for treatments in lung, breast, and colorectal cancer.
• Monitoring response: Repeated blood draws during treatment let oncologists track whether ctDNA levels are falling, rising, or changing in composition — a real-time signal of how well therapy is working.
• Detecting recurrence: After treatment ends, liquid biopsies can detect ctDNA months or even years before new tumors become visible on imaging scans, according to the National Cancer Institute.

The Screening Frontier

The most ambitious goal is using liquid biopsies to screen healthy people for cancer before symptoms appear. In 2024, the FDA approved Shield, the first blood-based screening test for colorectal cancer in average-risk adults. It detects ctDNA with roughly 83% sensitivity for colorectal cancer, though its ability to catch precancerous polyps remains much lower — around 13%, according to AAMC.

Several companies are developing multi-cancer early detection (MCED) tests designed to screen for dozens of cancer types simultaneously from a single blood draw. These tests analyze not only ctDNA mutations but also methylation patterns — chemical tags on DNA that change when cells become cancerous.

Key Limitations

Liquid biopsies are not yet a replacement for traditional tissue biopsies in every scenario. The fundamental challenge is sensitivity: early-stage tumors shed very little DNA into the blood, making detection difficult. As researchers at the NCI have noted, insufficient ctDNA in the blood remains a primary reason liquid biopsy tests miss cancers.

False positives also pose a concern. Age-related mutations in blood cells — a phenomenon called clonal hematopoiesis — can mimic cancer signals, potentially triggering unnecessary follow-up procedures. Cost is another barrier: many liquid biopsy tests run into thousands of dollars, and insurance coverage remains inconsistent outside of FDA-approved companion diagnostics.

Where the Field Is Heading

Despite these hurdles, liquid biopsies represent one of the fastest-moving areas of cancer research. Researchers are working to boost sensitivity by collecting larger blood volumes, combining ctDNA analysis with protein biomarkers, and using artificial intelligence to detect subtle patterns. As the technology matures, the vision is straightforward: a routine annual blood test that catches cancer early enough to cure it.