No single blood test can reliably detect all cancers. Most cancers don’t show up in routine blood work at all. But several types of blood tests play important roles in cancer detection, from standard tumor markers that flag specific cancer types to newer multi-cancer detection tests that scan for dozens at once. The type of test that matters depends on whether you’re screening for cancer before symptoms appear, investigating a suspicious finding, or monitoring a known diagnosis.
Why Routine Blood Work Usually Misses Cancer
A standard complete blood count (CBC) or metabolic panel isn’t designed to catch cancer, and it won’t detect solid tumors like breast, lung, or colon cancer. A CBC can pick up blood cancers like leukemia, lymphoma, and multiple myeloma because these diseases directly change the number or type of cells circulating in your blood. Abnormally high or low white blood cell counts, unexpected anemia, or unusual platelet levels can all raise suspicion.
Blood protein tests, sometimes included as part of a comprehensive metabolic panel, can also hint at blood cancers. Elevated protein levels in the blood may point toward multiple myeloma or lymphoma. But these findings are nonspecific. Many conditions besides cancer cause similar results, which is why abnormal routine blood work leads to further testing rather than a diagnosis on its own.
Tumor Marker Tests
Tumor markers are substances, usually proteins, that certain cancers release into the bloodstream. Doctors order these tests when they suspect a specific cancer type or need to track how well treatment is working. The most commonly used tumor marker tests include:
- PSA (prostate-specific antigen): Used for prostate cancer screening and monitoring. Elevated levels can also result from non-cancerous prostate enlargement, so a high PSA doesn’t automatically mean cancer.
- CA-125: Associated with ovarian cancer. Often used to monitor treatment response rather than for initial screening, since other conditions like endometriosis can raise levels.
- CEA (carcinoembryonic antigen): Linked to colorectal cancer and some other cancers. Frequently used to monitor whether colorectal cancer has returned after treatment.
- AFP (alpha-fetoprotein): Connected to liver cancer, certain ovarian cancers, and germ cell tumors.
- CA 19-9: Associated with pancreatic, gallbladder, bile duct, and stomach cancers.
- CA 15-3 / CA 27.29: Used in breast cancer monitoring.
- Thyroglobulin and calcitonin: Used for different types of thyroid cancer.
Tumor markers have a significant limitation: they’re better at monitoring known cancers than at finding new ones. Many of these markers can be elevated for reasons unrelated to cancer, and some cancers don’t produce detectable markers at all. A normal result doesn’t rule cancer out, and an abnormal result doesn’t confirm it. That’s why these tests are almost always interpreted alongside imaging, biopsies, or other diagnostic tools.
Multi-Cancer Early Detection Tests
A newer category of blood test aims to screen for many cancers simultaneously. These multi-cancer detection (MCD) tests look for biological signals that tumors shed into the bloodstream, most commonly fragments of DNA released by cancer cells or chemical modifications on that DNA. Different tests screen for different groups of cancers. Some focus on cancers that already have established screening methods, like colorectal and breast cancer, while others target cancers that are notoriously hard to catch early, like ovarian and pancreatic cancer.
The most widely discussed MCD test, called Galleri, screens for more than 50 cancer types using a single blood draw. It works by analyzing patterns of DNA methylation, which are chemical tags on DNA fragments that differ between cancerous and healthy tissue. When the test detects a cancer signal, it also predicts where in the body the cancer likely originated.
The accuracy numbers tell an important story, though. Galleri’s overall sensitivity for early-stage cancers (stages 1 and 2) is only about 27.5%. That means it misses roughly three out of four early-stage cancers. When restricted to 12 cancers the test’s developers identified as having high unmet need, sensitivity improves to about 53%. The false-positive rate is low, around 0.5%, meaning the test rarely signals cancer when none exists. But the low sensitivity for early-stage disease means a negative result provides limited reassurance.
No multi-cancer detection test has received FDA approval. Some companies sell them directly to physicians and consumers as laboratory-developed tests, which face less regulatory oversight. The U.S. Preventive Services Task Force, the body that issues national screening guidelines, does not currently recommend any blood test for routine cancer screening in the general population. Standard screening recommendations still rely on mammography for breast cancer, low-dose CT scans for lung cancer in high-risk individuals, and stool-based tests or colonoscopy for colorectal cancer.
How Liquid Biopsies Work
The technology behind these newer blood tests falls under the umbrella of “liquid biopsy,” a term for any test that analyzes cancer-related material circulating in the blood. Two main targets make this possible.
The first is circulating tumor DNA (ctDNA). When cancer cells die, they release tiny fragments of their DNA into the bloodstream. These fragments carry the same genetic mutations and methylation patterns as the tumor itself. The challenge is that ctDNA makes up an extremely small fraction of all the free-floating DNA in your blood, so detection methods need to be extraordinarily sensitive. Techniques like digital droplet PCR can identify one mutant DNA molecule among 10,000 normal ones.
The second target is circulating tumor cells (CTCs), which are intact cancer cells that have broken away from a tumor and entered the bloodstream. These are even rarer than ctDNA, and isolating them requires specialized methods. One FDA-cleared system uses antibody-coated magnetic particles to capture tumor cells based on proteins on their surface, then confirms them under a microscope. CTC tests are used primarily in advanced cancers like metastatic breast, prostate, and colorectal cancer, where the number of circulating cells can help predict outcomes.
DNA Methylation: A More Sensitive Approach
One of the more promising developments in cancer blood testing involves analyzing methylation patterns, not just in tumor DNA floating in the blood, but in immune cells themselves. Cancer changes how your immune system behaves at a molecular level, and those changes appear early, sometimes before a tumor is large enough to shed detectable amounts of its own DNA.
A 2023 study in colorectal cancer illustrates the potential. Researchers built a diagnostic model based on five methylation markers found in immune cells from a standard blood draw. For early-stage colorectal cancer, this approach achieved 81% sensitivity and 89% specificity. By comparison, CEA, the traditional tumor marker for colorectal cancer, caught only 28% of early-stage cases at its standard threshold. When the methylation-based test was combined with CEA, sensitivity rose to 84% with 95% specificity. The test also detected advanced precancerous growths (advanced adenomas) with 63% sensitivity, a stage when intervention can prevent cancer entirely.
These results haven’t yet translated into a widely available clinical test, but they highlight why methylation-based approaches are attracting so much attention. They detect signals that appear earlier in cancer development than the protein markers doctors have relied on for decades.
What These Tests Can and Can’t Do
No blood test currently available can definitively diagnose cancer on its own. Every positive result from a blood test, whether it’s an elevated PSA, an abnormal CBC, or a signal on a multi-cancer detection test, needs confirmation through imaging, tissue biopsy, or both. Blood tests are best understood as one layer in a diagnostic process, not a standalone answer.
Where blood tests add the most value depends on the situation. For people with a known cancer, tumor markers and liquid biopsies help track whether treatment is working and detect recurrence early. For screening, the technology is advancing rapidly but isn’t yet accurate enough to replace established methods like mammography and colonoscopy. Multi-cancer detection tests may eventually fill a gap for cancers that currently have no screening option at all, like pancreatic and ovarian cancer, but their ability to catch these cancers at a curable stage is still limited.
If you’re considering a multi-cancer detection test, the most important thing to understand is what a result actually means. A negative result does not mean you’re cancer-free, especially for early-stage disease. A positive result will require follow-up testing, which may include imaging and procedures that carry their own costs, anxiety, and small risks. The tests are most useful when interpreted by a doctor who can weigh the result against your personal risk factors, family history, and symptoms.