The national blood inventory has dwindled so much that hospitals may have to delay treatments or postpone elective surgeries if the shortage is not reversed soon, Canadian Blood Services said Wednesday in urging donors to come forward.

The reserve dropped to two days on hand for the most common blood types on Tuesday, after falling more than 40 per cent in the last two months.

"It may surprise Canadians to know that even in good times, we only carry four to six days supply of blood to meet the entire country's needs, " said Dr. Graham Sher, CEO of Canadian Blood Services.

"Right now, for the most critical blood types, we are hovering around a two-day supply. This puts the blood system, and Canadians who depend on it, in a vulnerable position. Simply put, we need more donors now."

The agency has boosted recruitment, but demand has outpaced supply and blood shipments to hospitals have been rationed.

To increase donations from new and existing donors, the agency is calling on eligible Canadians to make an appointment to donate. The need for donors is greatest at the agency's 40 permanent locations across the country.

Emergency needs are being met, but the reduced availability of blood is challenging the ability of some hospitals to serve elective or routine patients, the agency said.

It is estimated that half of Canadians may give blood, but this year, one in 60 have done so.

In the last three years, hospital demand for blood is up 3.3 per cent, yet the number of donors has decreased by 0.7 per cent, according to the agency.

Canadian Blood Services aims to reserve blood in case of emergencies or natural disasters.

Héma-Québec runs Quebec's blood system.