There is no single blood test that definitively diagnoses cancer. Instead, doctors use several different types of blood tests depending on the suspected cancer, the stage of investigation, and whether the goal is screening, diagnosis, or monitoring treatment. These range from routine blood counts to specialized tests that search for DNA fragments shed by tumors.
Complete Blood Count
A complete blood count, or CBC, is one of the most common blood tests ordered in medicine. It measures your red blood cells, white blood cells, and platelets. Abnormal levels of any of these can signal blood cancers like leukemia and lymphoma, or suggest that a known cancer has spread to bone marrow. A CBC cannot detect solid organ cancers like breast, lung, or colon cancer on its own, but it’s often part of the initial workup when cancer is suspected because it reveals how well your blood and immune system are functioning.
Blood Protein Tests
Blood protein tests check for abnormal levels of specific proteins circulating in your blood. Unusually high protein levels can point toward blood cancers such as leukemia, lymphoma, and multiple myeloma. A comprehensive metabolic panel, which many people get during routine checkups, includes a total protein measurement that can flag these abnormalities. If protein levels come back elevated, further testing is needed to pinpoint the cause, since infections and other conditions can raise protein levels too.
Tumor Marker Tests
Tumor markers are substances that certain cancers produce in higher-than-normal amounts. A blood draw can measure these markers, and each one is linked to specific cancer types:
- PSA (prostate-specific antigen): Used for prostate cancer. The American Cancer Society notes that men with a PSA below 2.5 ng/mL may only need retesting every two years, while those at 2.5 or above should be tested annually.
- CA-125: Associated with ovarian cancer. Levels can also rise due to endometriosis or menstruation, so this test is more commonly used to monitor treatment response than to screen healthy women.
- CEA (carcinoembryonic antigen): Linked to colorectal cancer and some other cancers. It’s most useful for tracking whether treatment is working or whether cancer has returned after surgery.
- AFP (alpha-fetoprotein): Connected to liver cancer, ovarian cancer, and germ cell tumors.
An important caveat: tumor markers are rarely used alone to diagnose cancer. Many of them can be elevated by non-cancerous conditions, and some people with cancer have normal marker levels. These tests are most valuable for monitoring a known cancer over time, checking whether treatment is shrinking the tumor, or watching for recurrence after remission.
Liquid Biopsy
Liquid biopsy is a newer category of blood test that looks for physical traces of a tumor in the bloodstream. There are two main approaches. The first searches for circulating tumor DNA (ctDNA), which consists of short fragments of DNA that cancer cells shed as they go through their normal life cycle. The second captures whole circulating tumor cells (CTCs) that have broken away from a tumor and entered the blood.
Your body’s healthy cells also release DNA fragments into the bloodstream, called cell-free DNA. The key difference is that fragments from tumor cells tend to be shorter and carry genetic mutations specific to the cancer. Liquid biopsy tests can identify these mutations, which helps doctors choose targeted therapies and track how a cancer evolves during treatment without requiring a surgical biopsy.
Multi-Cancer Early Detection Tests
A newer class of blood tests aims to screen for dozens of cancers at once. The two main options currently available are Galleri and CancerGuard, both of which can screen for more than 50 cancer types from a single blood draw. These tests work by analyzing patterns in DNA fragments or other molecular signals in the blood to detect a cancer signal and predict where in the body it might be located.
Results from a large clinical trial showed an updated multi-cancer blood test had a positive predictive value of 62%, meaning it correctly identified cancer in roughly six out of ten people who received a positive result. The false-positive rate was just 0.4%, which means very few healthy people received a false alarm. That same trial found the blood test detected seven times more cancers than standard screening methods alone.
These tests are not yet FDA-approved to diagnose cancer, and they are not covered by most insurance plans. They’re positioned as a complement to existing screening tools like mammograms and colonoscopies, not a replacement. A positive signal still requires follow-up imaging and often a tissue biopsy to confirm whether cancer is actually present.
What Happens After an Abnormal Result
No blood test alone confirms a cancer diagnosis. If any of the tests described above come back abnormal, the next steps typically involve imaging (such as a CT scan, MRI, or ultrasound) to locate and measure any suspicious area, followed by a tissue biopsy if something concerning is found. The biopsy, where a small sample of tissue is examined under a microscope, remains the gold standard for confirming cancer and determining its type and stage.
Blood tests play a critical role at every phase of the cancer journey: flagging early warning signs, helping narrow down a diagnosis, guiding treatment decisions, and monitoring for recurrence years after treatment ends. But they work as one piece of a larger diagnostic picture, not as a standalone answer.