… could show up. For example, if you feel like crap and show up to some ED and your white blood cell count is through the roof (with maybe a deficit in RBCs and platelets), you might suspect a leukemia or lymphoma. However, that constellation of labs could also be the result of something infectious.

In order to confirm a suspicion of a liquid malignancy, you then send a FISH test on the blood (fluorescence in situ hybridization). This will take a look at DNA issues typical of various blood cancers and can provide a definitive diagnosis.

If you are thinking multiple myeloma, you're gonna do a protein electrophoresis. Myeloma will cause certain proteins to be present in the blood stream.

Solid tumors are even less related to standard blood tests. Some posit that platelets which are ELEVATED could be preliminary indicators of a cancer -- but that's faaaarrrrrr from certain. Now, a number of solid tumors do cause "tumor markers" to enter the blood stream -- but you have to actively go out and look for those.