Yes, blood tests for cancer exist, but they come in different forms and serve different purposes. Some have been used for decades to monitor known cancers or flag specific types, while a newer category of multi-cancer early detection (MCED) tests claims to screen for dozens of cancer types from a single blood draw. None of these screening tests have FDA approval yet, and their ability to catch cancer early is still limited.
What Cancer Blood Tests Actually Detect
Cancer cells behave differently from normal cells at the molecular level, and some of those differences show up in your blood. The most established approach uses protein-based biomarkers: substances that certain cancers release into the bloodstream. PSA for prostate cancer and CA-125 for ovarian cancer are two well-known examples that doctors have used for years, though these are typically ordered when cancer is already suspected or being monitored during treatment, not as a general screening tool for healthy people.
The newer generation of tests works differently. These look for fragments of DNA that tumors shed into the bloodstream, called circulating tumor DNA. What makes this DNA identifiable isn’t just mutations. Cancer cells also carry distinct chemical tags on their DNA, called methylation patterns, that differ from healthy cells. These patterns are actually more consistent across cancers than mutations are, which tend to occur at unpredictable locations in the genetic code. By reading these chemical signatures, a blood test can potentially detect that cancer is present and even predict where in the body it’s growing.
Multi-Cancer Early Detection Tests
The most talked-about tests right now are MCED tests, which aim to screen for many cancer types at once. The Galleri test and CancerGuard test are both commercially available, meaning a doctor can order them for you. They are sold as lab-developed tests under federal laboratory regulations, but neither has been cleared or approved by the FDA.
These tests are designed to complement, not replace, existing cancer screenings like mammograms and colonoscopies. Their appeal is that they target cancers for which no standard screening exists: pancreatic, liver, ovarian, and others that are typically caught late, when treatment options are limited.
The amount of tumor DNA in your blood corresponds to how much cancer is present. This creates a fundamental challenge: small, early-stage tumors release very little DNA, making them harder to detect. The Galleri test has an overall sensitivity of just 27.5% for stage 1 and 2 cancers. That means it misses roughly three out of four early-stage cancers. When the analysis is narrowed to 12 cancer types with the greatest unmet need (cancers that lack good screening options), sensitivity improves to about 52.8%, but that still means nearly half go undetected.
The test performs better at not producing false alarms. Its reported false-positive rate is 0.5%, meaning 99.5% of people without cancer will correctly receive a negative result.
What Happens if the Test Is Positive
A positive MCED result is not a cancer diagnosis. It’s a signal that requires follow-up, and that follow-up can be extensive. You may need additional blood work, imaging scans like CT or MRI, endoscopy, and possibly biopsies to confirm whether cancer is actually present and where it is.
This process has been described as a “diagnostic odyssey.” Because the blood test may indicate a general region of the body but can’t pinpoint a specific tumor, doctors sometimes need to investigate multiple areas before confirming or ruling out cancer. Each additional procedure adds time, cost, and its own risks. For some people, the process ends with no cancer found at all, which brings relief but also means weeks or months of anxiety and medical procedures that weren’t ultimately necessary.
The positive predictive value of these tests, meaning the chance that a positive result actually reflects real cancer, varies widely depending on the test’s specificity and how long cancers remain detectable before causing symptoms. Modeling studies estimate this value could range from about 16% to 78%. At the lower end, that means for every true cancer detected, there could be more than five false positives leading to unnecessary workups.
The Problem of Overdiagnosis
Not every cancer that can be found needs to be found. Some cancers grow so slowly that they would never cause symptoms or shorten your life. Detecting these “indolent” cancers through screening leads to overdiagnosis: a real cancer is identified, but treating it provides no benefit because it was never going to be a threat. The person still undergoes surgery, radiation, or other treatments with real side effects for a disease that would have remained harmless.
Modeling research estimates that 2% to 6% of all cancers detected annually through multi-cancer screening would be overdiagnoses. That proportion climbs sharply with age. At age 50, roughly 1% of screen-detected cancers fall into this category. By age 75, over 10% do. This makes sense biologically: older adults are more likely to die of other causes before a slow-growing cancer ever becomes a problem, so detecting it earlier doesn’t extend their life.
Cost and Insurance Coverage
These tests are expensive, and you’ll almost certainly pay out of pocket. The Galleri test costs $949 and CancerGuard costs $689, neither of which includes the blood draw fee. Most insurance plans do not cover MCED tests, and Medicare does not either. Without FDA approval and formal screening recommendations from major medical bodies, there’s little incentive for insurers to start covering them soon.
The U.S. Preventive Services Task Force, which issues the screening guidelines that typically drive insurance coverage decisions, has not made any recommendation for or against MCED blood tests. Until large clinical trials demonstrate that these tests reduce cancer deaths (not just detect cancer), that recommendation is unlikely to change.
FDA-Approved Blood Tests for Cancer
It’s worth distinguishing screening tests from diagnostic and monitoring tests. The FDA has approved blood-based tests for specific, narrower purposes. One example is a liquid biopsy approved for identifying specific gene mutations in people already diagnosed with metastatic non-small cell lung cancer, helping doctors choose the right targeted therapy. These tests aren’t designed to tell you whether you have cancer. They help guide treatment for cancer you already know about.
Liquid biopsies are also increasingly used to monitor how a known cancer responds to treatment over time. Because a blood draw is far simpler and less invasive than a tissue biopsy, doctors can check more frequently whether a tumor is shrinking, growing, or developing new mutations that might require a change in therapy. This is a well-established and growing use of cancer blood tests, even though screening healthy people remains unproven territory.
Who Might Consider an MCED Test
People at elevated cancer risk sometimes pursue MCED testing as an additional layer of screening on top of standard tests. This might include individuals with strong family histories of cancer, certain genetic predispositions, or a personal history of cancer. Some cancer centers have set up dedicated clinics for patients who receive positive MCED results, which can help streamline the diagnostic follow-up process.
For the average person at normal risk, the value proposition is less clear. The low sensitivity for early-stage cancers means a negative result shouldn’t be reassuring enough to skip your regular mammogram, colonoscopy, or other proven screenings. And the cost, combined with the possibility of a stressful false positive, means the decision is worth careful thought. The technology is advancing quickly, but right now these tests work best as a supplement to established screening, not a replacement for it.