Liquid Biopsy Explained: How a Blood Test Is Replacing the Scalpel

Liquid biopsy is the most important diagnostics advance in oncology since the CT scan. Instead of cutting out tumor tissue with a needle, oncologists can detect cancer DNA floating in a patient’s blood. The market is growing at double-digit rates. Every new targeted cancer drug pulls a companion diagnostic along with it. If you invest in biotech, you need to understand liquid biopsy.

What is a liquid biopsy?

A liquid biopsy is a blood draw that detects three things: circulating tumor DNA (ctDNA) shed by tumors into the bloodstream, circulating tumor cells (CTCs) that have broken off from the primary tumor, and exosomes (small vesicles carrying tumor molecular information).

The science in plain English: tumors are messy. They shed DNA into the blood the way a factory sheds smoke. A liquid biopsy reads that DNA to tell you what mutations the tumor carries, whether the tumor is still present after surgery, and whether the cancer is coming back before imaging can see it.

Why it matters versus traditional biopsy

Traditional tissue biopsy requires a needle, sometimes surgery, and carries infection and bleeding risk. It can be done once or twice. Liquid biopsy is a blood draw. It can be repeated monthly. It detects recurrence months before imaging. It works for cancers where tissue biopsy is impossible (lung cancers near major blood vessels, pancreatic cancers behind the bowel).

The three main use cases:

1. Treatment selection: Testing for mutations (EGFR, KRAS, ALK) to match patients to targeted drugs. If you have an EGFR mutation, you get Tagrisso. If you do not, you do not. The liquid biopsy determines the treatment.

2. Minimal residual disease (MRD) monitoring: After a surgeon removes a tumor, they cannot see if microscopic cancer remains. A liquid biopsy can detect one cancer DNA molecule in 100,000 normal DNA molecules. If MRD is positive after surgery, the oncologist gives chemotherapy. If negative, they may spare the patient the toxicity. For a real example, see our Natera Signatera analysis.

3. Early detection and screening: Finding cancer in healthy people before symptoms appear. Grail’s Galleri test screens for over 50 cancer types from one blood draw. Exact Sciences’ Cologuard screens for colorectal cancer from a stool sample.

The key players

Natera (NTRA) closed at $273.87 with a market cap of $39.4 billion. Natera is the MRD leader with approximately 80% U.S. market share in test volume. Signatera, their MRD test, is used in colorectal, breast, and lung cancer monitoring. The company has published over 100 peer-reviewed studies validating the test. The growth story is real: Natera’s test volume grew over 50% year over year in the most recent quarter.

Guardant Health (GH) is the leader in treatment selection. Guardant360 is the most-ordered liquid biopsy for matching patients to targeted therapies. Guardant’s Shield test, approved by the FDA in 2024 for colorectal cancer screening, is the company’s entry into the screening market.

Exact Sciences (EXAS) owns Cologuard (colorectal cancer screening) and Oncotype DX (breast cancer recurrence scoring). The company is expanding into MRD through acquisition.

Grail (now independent from Illumina) operates the Galleri multi-cancer early detection test. Galleri is the most ambitious screening product: one blood draw, over 50 cancers. The clinical evidence is still building, and reimbursement is uncertain.

The reimbursement risk

This is the binary risk in diagnostics that biotech investors ignore until it hits. CMS (Medicare) determines what the government pays for diagnostic tests. A reimbursement cut can gut the growth story overnight. Natera’s Signatera is currently reimbursed under a local coverage determination (LCD) that sets the price at roughly $2,300 per test. If CMS were to cut that price by 50%, Natera’s revenue projections would collapse.

This is not hypothetical. Exact Sciences fought a multi-year battle to get Cologuard’s screening age expanded from 45 to 50 and to increase the reimbursement rate. Guardant has had to fight for Shield coverage. Reimbursement is the structural risk in diagnostics investing.

The companion diagnostic flywheel

Every new targeted cancer drug approval creates a companion diagnostic market. When the FDA approves a new EGFR inhibitor, every patient with EGFR-mutant lung cancer needs a test to confirm their mutation. The drug company sells the drug. The diagnostics company sells the test. The two businesses grow together.

This is why liquid biopsy companies benefit from the oncology pipeline. Every new targeted therapy from AstraZeneca, Pfizer, Merck, and Roche pulls a diagnostic test along with it. For context on how companion diagnostics work with cancer drugs, see our checkpoint inhibitor explainer and our ADC technology guide.

How to value diagnostics companies

Diagnostics companies are valued differently from therapeutic biotech. They have recurring revenue (every follow-up test is a new sale), lower binary risk (a single test failure does not zero the company), but lower upside per product (a diagnostic test sells for $2,000, a cancer drug sells for $100,000 per year).

Price-to-sales multiples matter more than pipeline optionality. Natera trades at roughly 15x revenue. Guardant trades at roughly 8x revenue. These are premium multiples justified by growth rates, but they leave no room for reimbursement disappointment.

The verdict

Liquid biopsy is a real technology with real clinical utility and real revenue growth. The risks are reimbursement and competition. Natera is the best-positioned company in the space, with dominant MRD market share and a validated test. At $39.4 billion, the stock prices in continued 50% growth and stable reimbursement. Both of those assumptions carry risk.

The diagnostics sector does not have the binary upside of a pre-PDUFA biotech, but it has more predictable revenue and lower clinical risk. For investors who want oncology exposure without the binary risk of a single drug approval, liquid biopsy is the cleanest way to play it. For a broader diagnostics sector view, see our diagnostics and liquid biopsy roundup.

Source: Natera Signatera clinical evidence