BTW, a "tumour" (or "tumor") is just an unusual mass of tissue. Not all tumours are cancerous.
A full blood count (FBC) measures around 15 different parameters of the patient's blood. Each parameter has a "normal" range, and hence a parameter can therefore be normal, too low, or too high. Depending on which parameters are out of normal range, the doctor can get some significant clues about what's happening in the body; however, these are generally clues rather than a specific diagnosis. More tests will be needed.
If you have cancer, it is quite possible that your FBC will be abnormal, however the doctor would still need to make more tests and eliminate a number of possible alternative diagnoses. I think that for most cancers, any tumour would need to be imaged using X-Ray/MRI/CT/Ultrasound and confirmed as cancer by means of a biopsy sample.
So, to sum up. There are many possible diseases that can cause problems with your FBC, and cancer is just one possibility.
