Does having a very high white cell count mean that I have blood cancer?

Not necessarily, though the context of the high white cell count needs to be further explored, ideally by a specialist haematologist. Key areas of assessment include how high is the count, what type of white cells are elevated (differentials), how long has it been elevated, is the rest of full blood examination normal, and last but not the least, does the person has any concerning symptoms. Thorough history taking, examination and further blood tests and/ or imaging are often required. It is very uncommon that a bone marrow biopsy would be needed in the first instance.