No single blood test can definitively diagnose most cancers, but several types of blood tests can reveal warning signs, help narrow down a diagnosis, or track how well treatment is working. The specific test your doctor orders depends on what type of cancer is suspected. Here’s what each major category of blood test looks for and what the results actually mean.
Complete Blood Count (CBC)
A complete blood count is often the first blood test ordered when cancer is a possibility. It measures your levels of red blood cells, white blood cells, and platelets. On its own, a CBC cannot confirm cancer, but certain patterns raise a red flag. Low red blood cells (anemia), low platelets, or an unusually high white blood cell count can all point toward blood cancers like leukemia or lymphoma.
A CBC is useful because it gives doctors a snapshot of how your bone marrow is functioning. Leukemia, for example, causes the bone marrow to produce large numbers of abnormal white blood cells, which crowd out healthy cells. That’s why a person with undiagnosed leukemia might show up with sky-high white blood cells but dangerously low platelets or red blood cells at the same time. If a CBC looks suspicious, further testing, usually a bone marrow biopsy, confirms the diagnosis.
CBCs are also used during cancer treatment to monitor how chemotherapy is affecting your blood cell production, since many treatments temporarily suppress the bone marrow.
Tumor Markers
Tumor markers are proteins or other substances that certain cancers release into the bloodstream. Doctors test for specific markers depending on the type of cancer they suspect. The most commonly used ones include:
- PSA (prostate-specific antigen): Used for prostate cancer. A level above 4.0 ng/mL is generally considered abnormal, though doctors sometimes use a lower cutoff of 2.5 ng/mL for younger men and a higher one for older men. PSA testing has a significant false-positive rate: about 6% to 7% of men get a false positive on any given screening round, and only about 25% of men who go on to have a biopsy because of an elevated PSA actually have cancer.
- CA-125: Primarily associated with ovarian cancer. Elevated levels can also occur with endometriosis, fibroids, or even pregnancy, so this marker is more useful for monitoring treatment response than for initial screening.
- CEA (carcinoembryonic antigen): Linked to colorectal cancer and some other cancers. Normal levels fall between 0 and 3 ng/mL, but if you smoke, levels up to 5 ng/mL can be normal. CEA is most commonly used to watch for cancer recurrence after treatment rather than to catch cancer the first time.
- AFP (alpha-fetoprotein): Associated with liver cancer, ovarian cancer, and germ cell tumors.
An important limitation: tumor markers can be elevated for reasons that have nothing to do with cancer. Infections, inflammation, and benign conditions can all push these numbers up. That’s why an abnormal tumor marker almost always leads to additional testing, like imaging or a biopsy, rather than a cancer diagnosis on its own. These markers tend to be most valuable after a cancer diagnosis, where rising or falling levels tell doctors whether treatment is working.
Blood Protein Testing
A test called serum protein electrophoresis separates the proteins in your blood based on their size and electrical charge. Doctors are looking for an abnormal spike in a specific type of protein called an M-protein. When blood plasma cells become cancerous, they produce large quantities of a single, identical antibody. This shows up on the test as a sharp, distinctive peak that stands out from the normal protein pattern.
Finding an M-protein spike can point to multiple myeloma, a cancer of the plasma cells in bone marrow. It can also indicate related conditions that range from relatively harmless (a precancerous state called monoclonal gammopathy of undetermined significance) to more aggressive diseases. If the standard electrophoresis test looks normal but myeloma is still suspected, a more sensitive technique called immunofixation can pick up smaller amounts of abnormal protein that the first test might miss.
LDH and General Tissue Damage
Lactate dehydrogenase, or LDH, is an enzyme found in nearly every cell in your body. When cells are damaged or destroyed, they release LDH into the bloodstream. A high LDH level doesn’t point to any one disease. It simply tells doctors that significant tissue damage is happening somewhere.
In oncology, LDH is used to estimate how aggressively a cancer is growing. Higher levels suggest a greater “tumor burden,” meaning more cancer cells are present and actively turning over. It’s particularly relevant in lymphoma, melanoma that has spread, multiple myeloma, testicular cancer, and leukemia. Doctors also track LDH during chemotherapy: if levels drop, treatment is likely working. If they stay elevated or rise, it may signal that the current approach isn’t effective. LDH is a blunt instrument, but it’s a useful one for gauging the big picture.
Circulating Tumor DNA (Liquid Biopsy)
One of the newer tools in cancer blood testing involves detecting tiny fragments of DNA that tumors shed into the bloodstream. These tests, often called liquid biopsies, look for circulating tumor DNA (ctDNA) and are increasingly used in two important situations: monitoring treatment response and catching cancer recurrence early.
For patients on active treatment, doctors can draw blood at regular intervals and measure whether ctDNA levels are rising or falling. This can reveal whether a therapy is working before imaging scans show any change, which is especially helpful for patients whose tumors look stable on scans but may actually be responding or progressing at a molecular level. In some cases, ctDNA testing can also identify the specific genetic mutations driving a tumor’s growth, which helps doctors match patients to targeted therapies.
After surgery or other curative treatment, ctDNA testing can detect what’s called minimal residual disease, meaning a tiny number of cancer cells that remain in the body but are too few to show up on a CT or MRI scan. Across multiple studies, ctDNA has detected recurrence 7 to 16 months before conventional imaging would have caught it. That early warning window gives doctors a chance to intervene sooner. These tests are currently used most in lung cancer, colorectal cancer, and breast cancer, though their applications are expanding.
Circulating Tumor Cells (CTCs)
Separate from tumor DNA fragments, some tests look for whole cancer cells circulating in the bloodstream. The FDA has cleared a test that counts these circulating tumor cells in patients with metastatic breast, colorectal, or prostate cancer. The key threshold is 5 cells per 7.5 milliliters of blood. Patients with fewer than 5 CTCs at any point during treatment tend to have longer progression-free survival, while those with 5 or more face a much higher likelihood of rapid disease progression.
CTC counts are used primarily as a prognostic tool for people already diagnosed with advanced cancer. They help doctors and patients understand how the disease is likely to behave and whether the current treatment strategy is keeping things in check.
What Blood Tests Cannot Do
Blood tests are powerful tools for raising suspicion, monitoring known cancers, and guiding treatment decisions. But with very few exceptions, they cannot diagnose cancer by themselves. Most solid tumors, including breast, lung, and colon cancers, still require imaging and a tissue biopsy for a definitive diagnosis. Blood work is one piece of a larger puzzle. An abnormal result is a starting point, not a verdict, and a normal result does not guarantee that cancer is absent. The value of these tests comes from combining them with your symptoms, imaging findings, and clinical history to build a complete picture.