There is no single blood test that can reliably detect all types of cancer. Instead, doctors use a range of blood tests depending on the type of cancer suspected, the symptoms involved, and whether the goal is screening, diagnosis, or monitoring treatment. The most common category is tumor marker tests, which measure proteins or other substances that cancer cells (or your body in response to cancer) release into the bloodstream. Other approaches include protein profiling, circulating tumor DNA analysis, and newer multi-cancer detection tests still under development.
Tumor Marker Tests
Tumor markers are substances, usually proteins, that show up at higher-than-normal levels in your blood when certain cancers are present. Each marker is associated with specific cancer types. Some of the most widely used include:
- PSA (prostate-specific antigen): Used for prostate cancer screening. For men in their 40s and 50s, a level above 2.5 ng/mL is considered abnormal, with a typical reading around 0.6 to 0.7 ng/mL. For men in their 60s, the threshold rises to 4.0 ng/mL. A rapid increase of more than 0.35 ng/mL in a single year can also trigger further testing.
- CA-125: Primarily linked to ovarian cancer. Elevated in about 50% of early-stage ovarian tumors and 92% of advanced-stage cases.
- CEA (carcinoembryonic antigen): Most commonly used to monitor colorectal cancer, though it can be elevated in other cancers too.
- AFP (alpha-fetoprotein): Associated with liver cancer, certain ovarian cancers, and germ cell tumors.
- CA 19-9: Linked to pancreatic, gallbladder, bile duct, and stomach cancers.
- CA 15-3 and CA 27.29: Used in breast cancer monitoring.
Less common but still important markers include calcitonin for medullary thyroid cancer, thyroglobulin for other thyroid cancers, and chromogranin A for neuroendocrine tumors. Beta-hCG, well known as a pregnancy hormone, is also a marker for certain germ cell tumors and a rare cancer called choriocarcinoma.
Why Tumor Markers Aren’t Definitive
An elevated tumor marker does not automatically mean you have cancer. Noncancerous conditions, including infections, inflammation, liver disease, and even pregnancy, can raise the levels of many of these markers. PSA, for example, can spike after a urinary tract infection or an enlarged prostate. CA-125 can be elevated during menstruation, endometriosis, or pelvic inflammatory disease.
Because of this, tumor markers are rarely used alone to diagnose cancer. They’re most valuable in combination with imaging, biopsies, and physical exams. In many cases, their primary role is actually after a diagnosis: tracking whether treatment is working, catching a recurrence early, or monitoring how a known cancer is progressing over time.
Blood Protein Testing for Blood Cancers
For cancers that originate in the blood or bone marrow, a different type of blood test is more useful. Serum protein electrophoresis separates the proteins in your blood by size and electrical charge, creating a pattern that reveals abnormal protein production. In multiple myeloma, a cancer of plasma cells, this test typically shows a distinctive sharp spike called an M-protein spike in the gamma region of the results.
If the M-protein level falls between 1.5 and 2.5 grams per deciliter, doctors will usually order additional tests to quantify which types of immune proteins are involved and collect a 24-hour urine sample for further analysis. An M-protein level above 2.5 g/dL prompts a more extensive workup including bone imaging and additional blood markers. This kind of testing is specific to plasma cell disorders and wouldn’t be used for solid tumors like breast or lung cancer.
Liquid Biopsies and Circulating Tumor DNA
A newer category of blood test looks for tiny fragments of DNA that tumors shed into the bloodstream, known as circulating tumor DNA. These liquid biopsy tests can identify specific genetic mutations in a tumor without needing a traditional tissue biopsy, which involves a needle or surgery.
The FDA has approved several liquid biopsy tests as companion diagnostics, meaning they help match patients with specific targeted therapies. FoundationOne Liquid CDx, for instance, is approved to detect certain gene mutations in the blood of patients with metastatic prostate cancer and breast cancer, helping doctors determine which targeted drugs are likely to work. These tests are not used for initial cancer screening in healthy people. They’re designed for patients who already have a known or strongly suspected cancer and need guidance on treatment.
Multi-Cancer Early Detection Tests
The most ambitious development in cancer blood testing is the multi-cancer early detection (MCED) test. These tests use machine-learning algorithms to analyze DNA fragments and protein patterns in a blood sample, attempting to identify signals from dozens of cancer types at once and even predict where in the body the cancer originated. Some tests in development screen for over 50 tumor types from a single draw.
The concept is appealing, but there are significant limitations. None of these tests are currently approved by the FDA. One major concern is that early-stage tumors may not shed enough DNA into the bloodstream to be reliably detected, meaning the tests could miss the very cancers they’re designed to catch early. A positive result still requires follow-up imaging and biopsy to confirm whether cancer is actually present, and a negative result doesn’t guarantee you’re cancer-free.
These tests are available through some doctors on a self-pay basis, but they are not yet part of standard cancer screening guidelines. They may eventually complement existing screening methods like mammograms and colonoscopies rather than replace them.
Complete Blood Count and Basic Blood Work
A standard complete blood count (CBC) doesn’t detect cancer directly, but it can reveal abnormalities that point toward blood cancers like leukemia or lymphoma. Unusually high or low white blood cell counts, unexplained anemia, or abnormal platelet levels can all be early clues. A comprehensive metabolic panel showing unexpected changes in liver or kidney function might also prompt a doctor to investigate further, since some cancers affect organ function before causing obvious symptoms.
These routine tests are often where a cancer investigation begins, not because they’re designed to find cancer, but because something unexpected shows up during a regular checkup or evaluation for unrelated symptoms.
Which Test You Might Get and Why
The blood test your doctor orders depends entirely on the situation. For routine screening in people without symptoms, PSA testing for prostate cancer is the most established blood-based cancer screen, though guidelines vary on when to start and how often to repeat it. CA-125 is sometimes used for ovarian cancer screening in women at high genetic risk, but it’s not recommended for the general population because of its high false-positive rate.
If you already have symptoms or an imaging finding that raises concern, your doctor may order one or more tumor markers alongside other diagnostic tests. If you’ve already been diagnosed and are undergoing treatment, tumor markers become a regular part of follow-up blood work to track your response. And if you need help choosing a targeted therapy, a liquid biopsy may be the right test to identify the genetic profile of your tumor without an invasive procedure.
No blood test replaces a biopsy for a definitive cancer diagnosis. What blood tests do well is narrow the possibilities, guide the next steps, and provide ongoing monitoring once a diagnosis is established.