There is no single blood test that detects all cancers. Instead, doctors use a range of blood-based tests depending on the situation: some screen for a specific cancer type, others track how well treatment is working, and newer tests attempt to detect multiple cancers at once from a single blood draw. The most familiar example is the PSA test for prostate cancer, but the landscape has expanded significantly in recent years.
Tumor Marker Tests
Tumor markers are substances that cancer cells, or sometimes normal cells responding to cancer, release into the bloodstream. A blood draw can measure their levels, giving doctors clues about whether a cancer might be present or whether a known cancer is growing or shrinking. The most commonly used tumor markers include:
- PSA (prostate-specific antigen): Used for prostate cancer. The U.S. Preventive Services Task Force recommends that men aged 55 to 69 make an individual decision about PSA screening after discussing the trade-offs with their doctor. Men 70 and older are generally not recommended for routine screening.
- CA-125: Associated with ovarian cancer. It’s more often used to monitor treatment response than as a first-line screening tool, partly because levels can rise from noncancerous conditions too.
- CEA (carcinoembryonic antigen): Linked to colorectal cancer and some other cancers. Typically used to monitor recurrence after treatment rather than to screen healthy people.
- AFP (alpha-fetoprotein): Used for liver cancer, ovarian cancer, and germ cell tumors.
An important thing to understand about tumor markers: most of them are not reliable enough to diagnose cancer on their own. Elevated levels can come from infections, inflammation, or benign conditions. Likewise, some people with cancer have normal marker levels. These tests work best as one piece of a larger diagnostic picture, often alongside imaging or biopsy.
The Shield Test for Colorectal Cancer
In July 2024, the FDA approved the first blood test for primary colorectal cancer screening in people at average risk. Called Shield, it works by detecting specific changes in cell-free DNA floating in the bloodstream, changes that signal the presence of a tumor or precancerous growth in the colon.
In a clinical study of nearly 8,000 people, Shield detected colorectal cancer in 83% of participants who were confirmed to have it by colonoscopy. It performed especially well for more advanced disease: it caught roughly 100% of Stage II, III, and IV cancers. For Stage I cancers, sensitivity dropped to between 55% and 65%.
The test’s biggest limitation is with precancerous growths. It identified only 13% of advanced precancerous polyps, meaning 87 out of 100 people with precancer would get a negative result. The false positive rate was 10%, so about 1 in 10 people without cancer or precancer would receive an incorrect positive result and need follow-up testing, usually a colonoscopy.
Shield is designed as an option for people who might otherwise skip screening entirely, not as a replacement for colonoscopy. If the choice is between a blood test and no screening at all, the blood test catches a meaningful number of cancers that would otherwise go undetected.
Multi-Cancer Early Detection Tests
A newer category of blood test aims to screen for dozens of cancer types from a single sample. The most well-known is the Galleri test, which analyzes DNA methylation patterns, essentially chemical tags on DNA fragments shed by tumors into the blood. These methylation patterns can indicate not only whether cancer is present but also where in the body it likely originated.
The promise is enormous, but the current performance has significant gaps. Overall sensitivity for early-stage cancers (Stage I and II) is about 27.5%. When the analysis is limited to 12 cancer types the developers identified as having the greatest unmet screening need, sensitivity improves to roughly 53%. The false positive rate is low at 0.5%, meaning very few healthy people would receive an incorrect positive result.
No multi-cancer detection test is currently FDA-approved or recommended by major medical guidelines for routine screening. Insurance plans, including Medicare, do not cover them. If you want one, expect to pay around $900 out of pocket, and you may also face additional costs for follow-up diagnostic procedures if the result comes back positive.
How DNA Methylation Works for Detection
Beyond multi-cancer tests, researchers are refining methylation-based approaches for specific cancers. The basic idea is that cancer changes how genes are switched on and off, not just in tumor cells but in immune cells circulating through the blood. These changes happen early, sometimes before a tumor is large enough to cause symptoms or show up on imaging.
One colorectal cancer study identified five specific methylation markers in immune cells that could distinguish people with cancer from healthy individuals. The approach detected early-stage colorectal cancer with 81% sensitivity and 89% specificity, outperforming the traditional CEA tumor marker. It also showed the ability to flag cancer risk up to two years before conventional diagnostic methods caught it. For advanced precancerous growths (adenomas), it detected about 63% of cases, a substantial improvement over the Shield test’s 13%.
These methylation-based methods are still in research stages for most cancer types, but they represent the direction blood-based cancer detection is heading: catching disease earlier by reading molecular signals the immune system produces in response to a developing tumor.
What Liquid Biopsies Actually Measure
You may hear the term “liquid biopsy” used as a catch-all for blood-based cancer tests. A traditional biopsy removes a piece of tissue with a needle or surgery. A liquid biopsy instead analyzes cancer-related material circulating in your blood. This includes two main targets.
The first is circulating tumor DNA (ctDNA), tiny fragments of DNA released by dying tumor cells. Labs use DNA sequencing to look for cancer-associated mutations in these fragments. The Shield and Galleri tests both fall into this category. The second is circulating tumor cells (CTCs), whole cancer cells that have broken away from a tumor and entered the bloodstream. These are far rarer and harder to find, requiring specialized techniques to isolate individual cells from billions of normal blood cells. CTCs are mainly used in research and for monitoring advanced cancers, not for screening.
Liquid biopsies are increasingly used after a cancer diagnosis to track treatment response, detect recurrence early, and identify specific mutations that might respond to targeted therapies. For screening healthy people, they remain a work in progress.
The False Positive Problem
Any screening test carries the risk of a false positive: a result that says cancer may be present when it isn’t. This matters because a positive blood test typically leads to imaging, additional blood work, and sometimes invasive procedures like biopsies or surgery.
The consequences are not trivial. In ovarian cancer screening research, false positive rates ranged from 0.1% to over 23% depending on the screening method and the patient’s risk level. When multiple screening tools were combined for high-risk individuals, the false positive rate reached 19.3%, leading to surgical interventions in about 5% of those screened. In average-risk populations, even a lower false positive rate of 5% translated to 1,125 people undergoing surgery they did not need in one large trial. Complications from those surgeries included bowel injury, significant blood loss, and perforation.
This is why doctors weigh the benefits of cancer blood tests against the real harms of unnecessary follow-up. A test with high sensitivity but poor specificity can create more problems than it solves, particularly when used in large populations where cancer is relatively uncommon.
What Blood Tests Cannot Do
No blood test can definitively diagnose cancer. A positive result on any screening blood test is a signal that further investigation is needed, usually imaging followed by a tissue biopsy if something suspicious is found. The biopsy remains the only way to confirm a cancer diagnosis.
Blood tests also vary widely in what they can catch depending on cancer type, stage, and the specific biology of an individual’s tumor. Some cancers shed very little DNA or produce no measurable markers in early stages. Others produce markers that overlap with benign conditions, muddying the results. The technology is improving rapidly, but the current reality is that blood tests work best as one tool among several, not as a standalone answer.