Yes, there are blood tests that can detect cancer, but no single blood test reliably finds all types. Some blood tests have been part of standard cancer care for decades, used mainly to monitor known cancers or screen for specific types like prostate cancer. A newer category called liquid biopsies analyzes tiny fragments of tumor DNA or whole tumor cells circulating in your bloodstream, and multi-cancer early detection tests aim to screen for dozens of cancer types from a single blood draw. Each approach has real strengths and significant limitations worth understanding.
Traditional Tumor Marker Tests
The most established cancer blood tests measure proteins that certain tumors release into the bloodstream. The National Cancer Institute lists several that are in routine clinical use:
- PSA for prostate cancer, used for both screening and monitoring
- CA-125 for ovarian cancer, used to help with diagnosis and track recurrence
- AFP for liver cancer, ovarian cancer, and germ cell tumors
- CEA for colorectal cancer and some other cancers, mainly to track treatment and detect recurrence
- CA 19-9 for pancreatic, gallbladder, bile duct, and stomach cancers, used to assess treatment response
- CA 15-3 and CA 27.29 for breast cancer, used to check whether treatment is working or cancer has returned
- Calcitonin for medullary thyroid cancer
- Thyroglobulin for thyroid cancer, used to monitor treatment response and recurrence
Here’s the important distinction most people miss: the majority of these markers are not designed for initial cancer screening in healthy people. They’re used after a diagnosis to track how well treatment is working or to catch cancer coming back. PSA is one of the few that plays a role in screening, and even that remains a shared decision between you and your doctor because elevated PSA levels can also result from non-cancerous conditions like an enlarged prostate.
How Liquid Biopsies Work
Liquid biopsies represent a fundamentally different approach. Instead of measuring a protein that a tumor sheds, they look for the tumor’s own genetic material floating in your blood. As cancer cells grow, die, and divide, fragments of their DNA enter the bloodstream. These fragments, called circulating tumor DNA (ctDNA), tend to be shorter than normal DNA fragments that healthy cells release, which helps laboratory techniques distinguish between the two.
Some liquid biopsies go a step further and capture whole tumor cells circulating in the blood. These circulating tumor cells don’t just reveal that a tumor exists. They indicate the cancer may be progressing or spreading, since these intact cells are essentially the seeds that can establish new tumors in distant parts of the body.
Several liquid biopsy tests have received FDA clearance, but primarily as companion diagnostics. That means they’re approved to help choose the right treatment for someone who already has cancer, not to screen healthy people. For example, tests exist that analyze blood plasma from patients with non-small cell lung cancer to identify specific genetic mutations, which then determines whether certain targeted therapies will work. Similar blood-based tests are approved for leukemia and ovarian cancer treatment decisions.
Multi-Cancer Early Detection Tests
The most ambitious blood tests aim to screen for many cancers at once. These multi-cancer early detection (MCED) tests, the best known being Galleri, analyze patterns in DNA fragments found in your blood. Rather than looking for a single marker, they examine chemical modifications on the DNA to identify signatures associated with cancer and predict where in the body it might be located. Current versions report specificity around 99%, meaning that out of 100 people without cancer, roughly 99 will correctly get a negative result.
High specificity sounds impressive, but the real-world picture is more complicated. The largest trial of this technology, the NHS-Galleri study, enrolled more than 140,000 people between the ages of 50 and 77 in the UK starting in 2021. Results reported in early 2026 showed that the test did not meet its primary goal: it failed to produce a statistically significant reduction in late-stage cancer diagnoses compared to the control group. The company noted a “favorable trend” toward fewer advanced cancers for 12 tumor types, including lung, pancreas, colorectal, and ovarian cancers, but the data wasn’t strong enough to confirm the test works as hoped. Follow-up with participants is continuing.
No MCED test is currently FDA-approved for cancer screening in the general population. You can get some of these tests through your doctor, but they’re generally positioned as a supplement to standard screening methods like mammograms and colonoscopies, not a replacement.
Why Blood Tests Miss Some Cancers
Blood-based cancer detection faces several biological challenges that explain why it hasn’t replaced traditional screening. Early-stage tumors are small, and small tumors release very little DNA into the bloodstream. When the amount of tumor DNA is extremely low, it can fall below what the test is able to reliably detect, leading to a false negative result where you have cancer but the test says you don’t.
Some cancers are also known to release very little or no DNA into circulation regardless of their size, making them particularly hard to catch with liquid biopsies. On the flip side, false positives can occur when normal age-related changes in blood cells mimic cancer signals. A phenomenon called clonal hematopoiesis, where blood cells acquire mutations as you get older without becoming cancerous, is a recognized source of confusion for these tests. A test might also detect a real genetic mutation but misidentify its source. A specific mutation found in the blood could belong to a lung tumor, an undiagnosed thyroid cancer, a colon growth, or even a benign mole.
What’s Covered and What It Costs
Coverage varies widely depending on the type of test. Medicare Part B covers blood-based biomarker screening for colorectal cancer with no out-of-pocket cost if you’re between 45 and 85, show no symptoms of colorectal disease, and are at average risk. If the blood test comes back positive, Medicare also covers the follow-up colonoscopy. This test is covered once every three years.
For multi-cancer detection tests like Galleri, the situation is different. These tests are not yet standard of care, and most insurance plans, including Medicare, do not cover them. Out-of-pocket costs for MCED tests can run several hundred dollars or more. Companion diagnostic liquid biopsies used to guide treatment for a known cancer are more commonly covered by insurance, since they’re tied to specific FDA-cleared treatment decisions.
What a Blood Test Can and Can’t Tell You
A positive result on any cancer blood test is not a diagnosis. It’s a signal that further investigation is needed, typically imaging or a tissue biopsy where a sample of the suspicious area is examined under a microscope. A tissue biopsy remains the gold standard for confirming cancer. Blood tests can narrow down where to look and what to look for, but they can’t replace that confirmation step.
A negative result also doesn’t guarantee you’re cancer-free. Given the limitations of current technology, particularly for early-stage disease, standard screening methods remain essential. PSA testing for prostate cancer, mammograms for breast cancer, colonoscopies or stool-based tests for colorectal cancer, and low-dose CT scans for lung cancer in high-risk individuals are still the foundation of cancer screening guidelines from organizations like the American Cancer Society.
Researchers at Johns Hopkins have developed a newer approach that measures the randomness of DNA chemical patterns rather than the patterns themselves. In early testing, this method detected the earliest stage of lung cancer with 81% sensitivity and early-stage breast cancer with about 68% sensitivity, both at 95% specificity. These are proof-of-concept results, not something available in a clinic yet, but they illustrate the direction the technology is heading: toward catching cancers earlier and across more types from a simple blood draw.