Blood tests alone can rarely, if ever, show the presence or absence of cancer.
Even, cancer blood tests, can't tell whether you have cancer or some other noncancerous condition, they can give your doctor clues about what's going on inside your body.
There is a test known as erythrocyte sedimentation rate, a commonly performed laboratory test along with regular blood count.
Malignancy (presence of cancer), and, some other conditions can cause elevated ESR.
Decreased number of RBCs (anemia), can be seen in colon cancer patient, due to colorectal bleeding.
Sometimes, the cancer itself can lead to detecting low white blood cells.
There are more specific blood tests, known as tumor marker tests that can detect certain tumor markers, which are proteins, antigens, or hormones, detected in higher than normal amounts in the blood, urine, or body tissues of some people with certain types of cancer.