The CBC lets you see how the dog is doing and if there are signs of infection or problems in the manufacturing of components of blood or if the dog is dehydrated etc.
http://www.cah.com/dr_library/labtests.html
While it wouldn't show cancer it would show if there is an issue with blood cell creation which might indicate a bone cancer or an increase immune system response by showing high white blood cell count etc.
Its a clue measured against norms and looked at in relationship to other medical signs not a total diagnosis.
Hope this helps you!
Please press Accept if you find my answer helpful. Want more info? Just reply on your question!
Bonuses for good work are always appreciated!
Thanks for the opportunity to help you!