No single blood test can definitively diagnose cancer on its own, but several types of blood tests play important roles in detecting, screening for, and monitoring the disease. These range from routine blood counts that can flag blood cancers to newer liquid biopsies designed to catch dozens of cancer types from a single draw. Which test matters depends on whether you’re screening for a specific cancer, tracking treatment, or looking for early warning signs.
Tumor Marker Tests
Tumor markers are proteins or other substances that cancer cells (or your body in response to cancer) release into the bloodstream. Doctors order these tests to help diagnose specific cancers, monitor whether treatment is working, or check if cancer has returned after treatment. They are not typically used as standalone screening tools for people without symptoms, because levels can be elevated for non-cancerous reasons too.
The most commonly used tumor markers include:
- PSA (prostate-specific antigen): Used to help diagnose prostate cancer and monitor for recurrence. This is the most widely known cancer-related blood test, though elevated PSA can also result from non-cancerous prostate enlargement or inflammation.
- CA-125: Associated with ovarian cancer. Helps with diagnosis, tracking treatment response, and watching for recurrence.
- CEA (carcinoembryonic antigen): Primarily used for colorectal cancer to check whether treatment is effective and to detect recurrence.
- AFP (alpha-fetoprotein): Linked to liver cancer, ovarian cancer, and germ cell tumors. Used both for diagnosis and for monitoring treatment.
- CA19-9: Connected to pancreatic, gallbladder, bile duct, and stomach cancers. Mainly used to assess whether treatment is working.
- CA15-3/CA27.29: Associated with breast cancer. Used to evaluate treatment effectiveness and detect recurrence.
Other markers exist for less common cancers. Calcitonin helps diagnose medullary thyroid cancer. Thyroglobulin tracks thyroid cancer after treatment. Chromogranin A is used for neuroendocrine tumors. Each marker corresponds to a specific cancer type, so doctors choose which to order based on what they suspect or what’s already been diagnosed.
An important limitation: tumor marker levels can fluctuate for reasons that have nothing to do with cancer. An elevated CA-125, for instance, can be caused by endometriosis or even menstruation. That’s why these tests are rarely used alone to make a diagnosis. They’re one piece of the puzzle alongside imaging, biopsies, and clinical evaluation.
Complete Blood Count
A complete blood count, or CBC, is one of the most basic and commonly ordered blood tests. It measures your red blood cells, white blood cells, and platelets. While it isn’t designed to screen for cancer, abnormal results can raise a red flag for blood cancers like leukemia and lymphoma.
In leukemia, white blood cell counts can climb dramatically. In children with acute leukemia, for example, an initial white blood cell count above 50,000 per microliter (normal is roughly 4,500 to 11,000) signals a more aggressive disease. But even more modest abnormalities matter. Unexplained drops in platelet counts, persistent anemia that doesn’t respond to iron supplements, or white blood cell counts that are unusually high or low can all prompt further investigation. Symptoms like unexplained bruising, frequent nosebleeds, or tiny red spots on the skin (petechiae) are clinical signs that doctors pair with CBC results to decide whether a hematologic cancer workup is needed.
A CBC won’t tell you whether you have cancer. But it’s often the first test that puts a doctor on alert, particularly for cancers that originate in the blood or bone marrow.
Multi-Cancer Early Detection Tests
A newer category of blood tests aims to detect many cancer types at once. These multi-cancer detection (MCD) tests analyze blood samples for fragments of DNA, RNA, or proteins shed by abnormal cells. The goal is to catch cancers early, including types that have no routine screening test today, like pancreatic or ovarian cancer.
The most well-known is the Galleri test, which screens for signals from more than 50 cancer types. It works by identifying abnormal DNA methylation patterns, which are chemical modifications on DNA that differ between cancer cells and healthy cells. When the test flags a potential cancer, it also predicts where in the body the signal is coming from, which helps guide follow-up testing.
The accuracy picture is mixed. The Galleri test has an overall sensitivity of about 27.5% for early-stage cancers (stages 1 and 2), meaning it catches roughly one in four early cancers. That number improves to about 52.8% when the analysis is limited to 12 cancer types the developers identified as having high unmet need, cancers that currently lack good screening options. On the other hand, the false-positive rate is low at around 0.5%, so a positive result is relatively unlikely to be a false alarm.
No MCD test has been approved by the FDA yet. Tests like Galleri and CancerGuard are available as lab-developed tests, which means doctors can order them, but they haven’t gone through the full FDA review process. Insurance typically won’t cover them, and most people pay out of pocket. The cost runs around $900, and you may also face additional costs for follow-up diagnostic procedures if the test flags something.
Blood Tests for Colorectal Cancer Screening
Colorectal cancer screening got its first FDA-approved blood test in July 2024. The Shield test detects colorectal cancer signals by identifying circulating tumor DNA in the bloodstream. It’s approved for adults 45 and older who are at average risk for colorectal cancer.
In clinical studies involving nearly 8,000 people, Shield detected colorectal cancer 83% of the time. That sounds promising, but there’s an important caveat: the test performs less well at catching advanced precancerous polyps, the growths that haven’t turned into cancer yet but could. A colonoscopy, by contrast, can find and remove those polyps before they ever become cancerous.
The National Comprehensive Cancer Network added Shield to its colorectal cancer screening guidelines but with a specific recommendation: it’s intended for patients who are unwilling to undergo screening through another method, like a colonoscopy or stool-based test. If you’re willing to do a colonoscopy, that remains the more thorough option. For people who would otherwise skip screening entirely, a blood draw every three years offers a meaningful alternative.
Liquid Biopsies and Circulating Tumor DNA
The technology behind many of these newer tests is called liquid biopsy. Instead of surgically removing tissue to examine under a microscope (a traditional biopsy), a liquid biopsy analyzes a standard blood sample for cancer-related material floating in the bloodstream.
Two main things are being detected. Circulating tumor DNA, or ctDNA, consists of tiny fragments of DNA that cancer cells release as they grow and die. These fragments carry the same genetic mutations found in the tumor itself, which allows the test to identify cancer-associated changes. Circulating tumor cells, or CTCs, are intact cancer cells that have broken away from a tumor and entered the blood. CTCs are harder to detect and are used less often in routine screening, but they offer a complementary window into what’s happening with a tumor at the cellular level.
Right now, ctDNA-based tests are more clinically developed. They’re used not just for early detection but also to guide treatment decisions in patients who already have cancer, helping oncologists choose targeted therapies based on the specific mutations present in a patient’s tumor. This avoids the need for repeated surgical biopsies, especially when a tumor is in a hard-to-reach location.
What These Tests Can and Can’t Do
Blood tests for cancer fall into two broad categories: those used for screening (finding cancer in people with no symptoms) and those used for monitoring (tracking known cancer or watching for its return). Most tumor markers fall into the monitoring category. MCD tests and the Shield blood test are pushing into screening territory, but with significant limitations.
No blood test available today can replace a biopsy for confirming a cancer diagnosis. A positive result on any of these tests leads to further workup, typically imaging and ultimately a tissue biopsy to confirm whether cancer is present and what type it is. A negative result doesn’t guarantee you’re cancer-free, especially with MCD tests that miss the majority of early-stage cancers.
Cost is a real barrier. Medicare does not currently reimburse for multi-cancer detection tests, and most private insurance plans follow the same approach. If you’re considering one of these tests, expect to cover the cost yourself and potentially pay for any follow-up procedures that result from an abnormal finding. For standard tumor marker tests ordered because of symptoms or a known cancer diagnosis, insurance coverage is generally not an issue since those are part of routine clinical care.