Cbc will not rule out cancer, unfortunately. Abnormalities in a cbc are very good for detecting various diseases, but do not always rule disease out when the results are normal. It will give an indication of whether or not you have leukemia, however, as leukemia causes abnormal production of blood cells. If blood cells are in the normal range, then leukemia can be ruled out. A cbc might also help determine if someone has lymphoma as results may be abnormal, but normal results would not rule out lymphoma. Cancer is variable in whether or not it is easily detectable, depending on the type. Usually symptoms may lead a doctor to begin exploring a potential cause of cancer, but cancer may not always present symptoms until late in the stage of the disease, and many other problems can mimic the symptoms of cancers. It's always best to be mindful of what the symptoms are and have them checked out if they pop up. Often, a biopsy of the abnormal growth (if present) is the only way to tell if one has many types of cancer.