About one-third of bookings for Newfoundland and Labrador blood drives don't show up, an official says. (CBC)

Would-be donors who forget or refuse to keep appointments at blood banks are contributing to a shortage in blood supplies, an official warns.

Canadian Blood Services issued a nationwide warning Wednesday that its reserves have dropped by about 40 per cent over the last two months. Instead of having enough blood to supply the country for six days, it currently has enough for two.

Some hospitals are even warning that elective surgeries will need to be postponed because of sharp drops in donations.

"We've seen fewer donors coming into our clinics," said Paul McGrath, CBS's regional communications manager for Atlantic Canada.

Contributing to the problem are "no-shows," or people who agree to come to a blood drive, but never make it. McGrath said the problem has become especially pointed in St. John's, where he works, and where the blood donation rate has plummeted.

"We do have the highest no-show rate in Canada right now, the highest number of donors who book an appointment and then don't show up — 35 per cent of our appointments are currently no-shows in Newfoundland and Labrador," McGrath said.

McGrath added that his heart sinks whenever he hears an ambulance, as he fears there will not be enough blood to handle a major accident.

The situation applies in communities across the country.

Halifax shortage monitored 'hour by hour'

"At this point our blood supply is really low. We have to monitor it, hour by hour," said Dr. Irene Sadek, who works at Halifax's Queen Elizabeth II Health Sciences Centre.

"We have to triage every blood request, for every patient, and make sure we're giving them the minimum that they need. "

Sadek told CBC News on Wednesday that the Queen Elizabeth may need to consider stopping some elective services so that hospital's blood supply remains stable.

Canadian Blood Services is urgently asking Canadians to make appointments to donate blood.

"At this point, the only remedy to the situation is to get more donors coming into our clinics," McGrath said in an interview.

Canadian Blood Services, which manages the blood system in all provinces except Quebec, said hospital demand for blood has increased by 3.3 per cent, while donations across the country have dropped by 0.7 per cent.

Héma-Québec manages Quebec's blood system.