What could cause a white blood cell count to be high but fluctuate up and down? My 1st test showed WBC of 20,000, the next week it dropped to 13,000. The last test it had went up to 16,9000. .

Not knowing your medical history, my answer is going to be broad. Now many vitamins, meds, and foods can cause this , like antibiotics are great for this. Or it could be an internal problem so you need to ask your DR about this to know for sure.  Good luck