Can a blood test detect cancer?

Blood tests can point toward cancer, but on their own they cannot confirm it or rule it out. The National Cancer Institute is clear that lab results are read alongside imaging and biopsies rather than in place of them.

The tests in question usually measure tumor markers. NCI defines a tumor marker as "anything present in or produced by cancer cells or other cells of the body in response to cancer or certain benign (noncancerous) conditions that provides information about a cancer."

Read that definition slowly. The marker can come from the cancer. It can also come from healthy cells reacting to something else entirely. That ambiguity is built into the biology, not into the laboratory.

The two ways a marker misleads

NCI names both failure modes plainly.

A raised result may mean nothing. "Having an elevated level of a tumor marker does not mean that someone has cancer. Noncancerous conditions can sometimes cause an increase in the level of a tumor marker."

A normal result may also mean nothing. Markers "often don't identify everyone with the disease (they are not sensitive enough)."

Sensitivity is a test's ability to catch the people who do have the condition. Specificity is its ability to clear the people who do not. Tumor markers tend to be middling at both. NCI's conclusion follows: "measurements of tumor markers are usually combined with the results of other tests, such as biopsies or imaging, to diagnose cancer."

A real example makes it concrete. NCI's PSA fact sheet reports that among men who have a prostate biopsy because of a raised PSA level, only about 25% turn out to have prostate cancer. Three of four do not.

Where blood tests genuinely earn their place

Not at the start. Later.

Once a cancer is known and its marker has been measured, that number becomes a tracking line. Falling markers during chemotherapy for testicular cancer suggest the drugs are working. A rising CA 19-9 after pancreatic surgery can flag a problem before symptoms do. PSA after prostate treatment does the same job.

The value comes from the trend in one person, not from a single reading in a healthy one. Even then, limits apply. NCI's pancreatic cancer summary for clinicians notes that "the presence of a normal CA 19-9 level does not preclude recurrence."

Blood tests also do plenty of ordinary work during treatment: counting white cells before a chemotherapy cycle, checking kidney and liver function, finding anemia.

About liquid biopsies

The field is moving. NCI explains that "tumors can shed cells or bits of biological material into blood, and these can be measured by tests called liquid biopsies." Multi-cancer early detection tests take this further, examining "DNA that tumor cells release into blood" and analyzing "multiple biomarkers in the blood of people without symptoms."

That is a real advance, and it is not settled science yet. NCI describes the true effectiveness of these tests as still under investigation.

So if you are offered one, ask a narrow question: what would you actually do with a positive result, and what would a negative one let you skip? Until those answers are solid, a blood test is an addition to proven screening such as mammography, colonoscopy, cervical screening, and low-dose CT for eligible smokers. It is not a substitute for any of them.