vWD testing on DDAVP?

My son just got tested last week and we are awaiting results. However, he currently uses DDAVP (desmopressin/stimate) for his diabetes insipidus. He uses the .01% Rhinal Tube and takes it orally (due to absorption issues) at .15mls BID. He has been using it for 5 yrs now. Do you think if he has been on this so long that it may produce a false negative for the vWD factor tests? I just worry that his bleeding with stools (not constipation) is related to having vWD but the test come back negative due to him taking the ddvap.

Also, he has had an EGD and colonoscopy. They came back normal and no tears, polyps, tumors, or anything.