many leukemias can be discerned from blood work, usually the white count is too low or high and then you fractionate it, or do a bone marrow biopsy when the blood count is low on all counts ( white, platelets, hematocrit) and you find myelodysplasia. CEA is a useful colon cancer to monitor tumor load but can't really be used to screen for colon cancer in it's earlier, curable stage when the CEA is not positive. Ditto for the CA-125, monitors tumor load but lousy for screening. PSA is unusually sensitive but not specific so many false positives. IF the alkaline phosphatase levels ( bone fraction) go up it can mean bony process or metastasis to bones, ( which is why it is part of routine blood work in anyone who has had a solid organ cancer that later likes to go to the bones like melanoma and breast cancer). The list goes on but they are ordered based on high pretest probability or clinical suspician since so many of these tests can be falsely positive in a non- cancer setting.