Can a blood test diagnose cancer?

A blood test alone usually cannot confirm cancer. According to the National Cancer Institute, high or low levels of certain substances in the body can be a sign of cancer, so lab tests of blood, urine, and other fluids can help doctors make a diagnosis. However, abnormal lab results are not a sure sign of cancer.

Lab results for healthy people vary from person to person, and even your own results can vary from day to day. For many tests, it is possible to have normal results even if you have cancer, and to have results outside the normal range even if you are healthy. This is why lab tests alone can't say for sure if you have cancer or another disease.

There are many types of blood tests used in cancer care, including the complete blood count (CBC), blood chemistry tests, tumor marker tests, and liquid biopsy, which looks for cancer cells or pieces of tumor DNA in the blood.

In most cases, doctors need to do a biopsy—removing a sample of abnormal tissue for a pathologist to examine—to be certain that a person has cancer. Your doctor is the best person to explain your lab results and what they mean for you.