No single blood test can definitively diagnose cancer, but several types of blood tests play important roles in detecting, screening for, and monitoring the disease. These range from routine blood counts that flag blood cancers to specialized tumor marker tests and newer multi-cancer detection technologies that scan for dozens of cancer types at once.
Complete Blood Count (CBC)
A complete blood count is the most basic blood test linked to cancer detection, and it’s the primary first step for diagnosing blood cancers like leukemia and lymphoma. A CBC measures your levels of red blood cells, white blood cells, and platelets. Abnormalities in any of these can signal a problem.
A very high white blood cell count, called leukocytosis, is one of the most common characteristics of leukemia. But the opposite can also be a red flag: abnormally low white blood cells make it harder for your body to fight infections and may indicate that something is disrupting normal blood cell production. When red blood cells, white blood cells, and platelets are all low at the same time (a condition called pancytopenia), that pattern is especially concerning and typically prompts further testing. Unusually high red blood cell levels can also point toward certain blood cancers and carry their own risks, including blood clotting.
A CBC alone doesn’t confirm cancer. Infections, nutritional deficiencies, and medications can all cause similar shifts. But persistent or unexplained abnormalities on a CBC are often what leads a doctor to order more targeted tests.
Tumor Marker Tests
Tumor markers are proteins or other substances that cancer cells, or sometimes normal cells responding to cancer, release into the bloodstream. Doctors use these tests primarily to monitor known cancers rather than to screen healthy people, though there are exceptions.
The most well-known tumor marker tests include:
- PSA (prostate-specific antigen): Used to help diagnose prostate cancer, track treatment response, and watch for recurrence. PSA is one of the few tumor markers sometimes used for screening in otherwise healthy men, though elevated levels can also result from non-cancerous prostate enlargement.
- CA-125: Associated with ovarian cancer. It helps with diagnosis, tracks how well treatment is working, and monitors for recurrence after treatment ends.
- CEA (carcinoembryonic antigen): Most commonly used for colorectal cancer. Doctors typically use it to check whether treatment is effective and to detect cancer that has come back.
- AFP (alpha-fetoprotein): Linked to liver cancer, ovarian cancer, and germ cell tumors. It helps with diagnosis, staging, and monitoring treatment response.
The key limitation of tumor markers is that they aren’t reliable enough to use as standalone screening tools for most cancers. Levels can be elevated for reasons that have nothing to do with cancer, and some people with cancer have normal marker levels. That’s why these tests are most valuable when combined with imaging or biopsy results, or when tracking changes over time in someone already diagnosed.
Liquid Biopsies and Circulating Tumor DNA
Liquid biopsy is a newer category of blood test that looks for tiny fragments of DNA shed by tumor cells into the bloodstream, known as circulating tumor DNA (ctDNA). Unlike traditional tumor markers, which detect proteins, liquid biopsies read the genetic fingerprint of a cancer.
Right now, ctDNA testing is used most often in people who already have advanced cancer. It helps doctors identify specific genetic mutations in a tumor without needing a tissue sample, which is especially useful when a tumor is hard to reach with a needle. Changes in ctDNA levels over time also correlate with how well a treatment is working. If levels drop, the therapy is likely effective. If new mutations appear, the cancer may be developing resistance, signaling that a treatment switch could be needed.
One of the most promising applications is detecting what’s called minimal residual disease. After surgery or treatment that appears to eliminate a cancer, trace amounts of ctDNA in the blood can reveal whether microscopic cancer cells remain. This could eventually help doctors decide who needs additional treatment after surgery and who can safely skip it. Research in this area is active but not yet standard practice for most cancer types.
Multi-Cancer Early Detection (MCED) Tests
Multi-cancer early detection tests represent the biggest shift in blood-based cancer screening. These tests analyze a single blood sample for signals associated with many different cancer types at once, often by looking at chemical patterns on cell-free DNA fragments circulating in the blood.
The most widely discussed MCED test, called Galleri, checks for abnormal DNA methylation patterns, which are chemical modifications that change how genes behave. Cancer cells produce distinctive methylation signatures that differ from normal cells, and the test uses these patterns to both detect cancer and predict where in the body it originated.
In clinical studies, the Galleri test showed an overall sensitivity of about 53% when focused on 12 cancer types with high unmet need, meaning cancers that currently lack effective screening methods. Its false-positive rate was reported at 0.5%, meaning that out of 1,000 people without cancer, roughly 5 would receive an incorrect positive result. Sensitivity improves significantly for later-stage cancers and is lower for early-stage disease, which is a meaningful tradeoff since early detection is where the test would provide the most benefit.
As of now, no MCED test has received full FDA approval for general cancer screening. Some are available as laboratory-developed tests that you can order through a doctor, but they are not yet covered by most insurance plans. These tests are designed to complement, not replace, existing cancer screening like mammograms and colonoscopies.
What Happens After an Abnormal Result
An abnormal blood test result is not a cancer diagnosis. It’s a starting point. The path from a flagged blood test to a confirmed diagnosis typically involves several additional steps.
Your doctor will likely begin with a physical exam and a review of your personal and family medical history. From there, imaging tests such as CT scans, MRIs, PET scans, or ultrasounds help create pictures of what’s happening inside the body and determine whether a tumor is present. These scans can narrow down the location and size of a suspicious area, but they still can’t confirm cancer on their own.
A biopsy is usually the only way to know for certain. During a biopsy, a doctor removes a small sample of tissue so a pathologist can examine it under a microscope. Depending on where the suspicious area is, the sample might be collected with a needle (common for breast, prostate, and liver), through an endoscope threaded into the body through a natural opening, or during surgery. The biopsy results determine whether cells are cancerous, what type of cancer is present, and often how aggressive it is.
This multi-step process means that even the most advanced blood tests serve as signals that prompt further investigation. They narrow the search and sometimes catch cancers earlier than symptoms alone would, but they work as part of a diagnostic chain rather than delivering a final answer by themselves.