Blood tests can play a role in detecting dozens of cancer types, but no single blood test catches them all, and different tests work in very different ways. Some measure specific proteins that certain tumors release into the bloodstream. Others look for fragments of tumor DNA floating in your blood. A routine complete blood count can flag blood cancers like leukemia. And newer multi-cancer screening tests claim to detect more than 50 cancer types from a single draw.
Understanding which test does what, and where the limitations are, helps you make sense of what a blood test can and can’t tell you about cancer.
Tumor Marker Tests for Specific Cancers
The most established cancer blood tests measure proteins, hormones, or other substances that specific tumors produce in abnormal quantities. These are called tumor markers, and they’ve been part of cancer care for decades. Most are used to monitor a known cancer or track whether it’s responding to treatment, not to screen healthy people. A few, like PSA for prostate cancer, are also used for screening in certain populations.
Here are the major tumor markers and the cancers they’re linked to:
- PSA: Prostate cancer. The most widely used screening marker, though elevated levels can also result from non-cancerous prostate enlargement.
- CA-125: Ovarian cancer. Often used to monitor treatment response rather than for initial screening, because levels can rise from other conditions.
- CEA: Colorectal cancer and some other cancers. Primarily used after diagnosis to track recurrence.
- AFP (alpha-fetoprotein): Liver cancer, ovarian cancer, and germ cell tumors.
- CA19-9: Pancreatic, gallbladder, bile duct, and stomach cancers.
- CA15-3 and CA27.29: Breast cancer, typically for monitoring metastatic disease.
- Calcitonin: Medullary thyroid cancer.
- Thyroglobulin: Thyroid cancer, mainly to detect recurrence after treatment.
- Beta-hCG: Certain pregnancy-related cancers and germ cell tumors.
An important caveat: most tumor markers are not reliable enough to diagnose cancer on their own. They can be elevated for non-cancerous reasons, and they can be normal even when cancer is present. That’s why they’re typically used alongside imaging, biopsies, and other diagnostic tools rather than as standalone tests.
Blood Cancers Found Through Routine Blood Work
Leukemia stands apart from solid tumors because the cancer itself lives in the blood and bone marrow. A standard complete blood count, the kind drawn during a routine physical, can raise the first red flag. People with leukemia typically show higher-than-normal white blood cell counts, and the ratios between different blood cell types often look abnormal.
If a CBC raises suspicion, additional tests like flow cytometry and a peripheral blood smear let doctors examine the blood cells more closely, identifying abnormal cell types that point toward a specific form of leukemia or lymphoma. This means blood cancers are sometimes caught incidentally during blood work ordered for an unrelated reason.
Multi-Cancer Early Detection Tests
A newer category of blood test aims to screen for many cancers at once. The most prominent example, Galleri, can detect more than 50 cancer types from a single blood draw. The list includes cancers that currently have no routine screening method: pancreatic, esophageal, stomach, liver, ovarian, kidney, and mesothelioma, among others.
These tests work by picking up fragments of DNA that tumors shed into the bloodstream, known as circulating tumor DNA. As cancer cells go through their life cycle, dying and dividing, they release short fragments of DNA into the blood. The test identifies these fragments and distinguishes them from the DNA that normal, healthy cells shed. Larger or more aggressive tumors tend to release more of this DNA, which means the test is generally better at catching later-stage cancers than very early ones.
Twelve of the cancers Galleri targets, including lung, colorectal, pancreatic, liver, ovarian, esophageal, and stomach cancers, account for roughly two-thirds of all cancer deaths in the United States. Many of these are cancers that have historically been diagnosed late, when treatment options are more limited, because no practical screening existed.
There are gaps, though. The test doesn’t detect cancers that don’t shed DNA into the bloodstream. Brain cancer, for example, is not reliably detected. And the test is not yet a replacement for established screening methods like mammography or colonoscopy.
The First FDA-Approved Colorectal Cancer Blood Test
In a significant milestone, the FDA approved the first blood-based screening tool specifically for colorectal cancer, called Shield. It marked the first time any blood test received approval as a primary screening option for colorectal cancer, and the first to qualify for Medicare coverage.
This matters because many people avoid colonoscopies. A simple blood draw could catch more cases in people who would otherwise skip screening entirely. That said, it’s considered a complement to existing options rather than a full replacement, since colonoscopy remains more sensitive and can remove precancerous polyps during the same procedure.
Genetic Blood Tests for Cancer Risk
A different category of blood test doesn’t look for cancer that’s already present. Instead, it looks for inherited gene mutations that dramatically increase your lifetime risk of developing certain cancers. These tests use a blood sample to check for changes in specific cancer-related genes passed down from a parent.
The most well-known examples are BRCA1 and BRCA2 mutations, which sharply increase the risk of breast and ovarian cancer. Lynch syndrome mutations raise the risk of colorectal, uterine, and several other cancers. Guidelines currently recommend genetic testing for all people with male breast cancer, ovarian cancer, pancreatic cancer, and metastatic prostate cancer. A positive result doesn’t mean cancer is inevitable, but it can guide more aggressive surveillance, preventive measures, and treatment choices. It also signals that close family members may benefit from testing.
False Positives and Real Limitations
No cancer blood test is perfect, and false positives are a real concern. A false positive means the test suggests cancer when none exists, which can trigger anxiety, additional imaging, and sometimes invasive procedures that carry their own risks.
The challenge is especially pronounced for screening in average-risk populations, where cancer is relatively rare. In ovarian cancer screening studies, false-positive rates varied enormously depending on the tool used, ranging from 0.1% to over 23%. Even a seemingly low false-positive rate of 5%, observed in one large trial of average-risk women, led to unnecessary surgeries in more than 1,100 people simply because the screened population was so large. Complications from those surgeries included perforation, significant blood loss, and bowel injury.
For a screening test to be practical in the general population, researchers estimate it needs a specificity of at least 99.6% and a sensitivity between 75% and 100%. No ovarian cancer screening study has yet met both thresholds. This is why most tumor markers remain monitoring tools rather than screening tools, and why newer tests like Galleri are still being evaluated in large clinical trials to determine whether early detection through blood actually improves survival.
The bottom line is that blood tests can surface signals from a wide range of cancers, from common ones like breast and prostate cancer to harder-to-catch types like pancreatic and esophageal cancer. But the technology varies widely in accuracy and purpose. Some tests screen for cancer before symptoms appear, some monitor cancers already diagnosed, and some assess inherited risk. Knowing which type you’re dealing with is the key to understanding what the results actually mean.