New guidelines introduced by Canadian Blood Services limiting the number of times female donors can donate in a year has the national agency scrambling to find new donors to make up the difference.

The change, implemented in December, was part of tougher guidelines on the amount of hemoglobin or iron required in the blood.

The new rules mean that female donors are now required to wait 12 weeks or 84 days between donations, as opposed to the previous eight weeks or 56 days.

Nothing has changed for male donors although as of March 5 individuals will have to pass a slightly higher hemoglobin test.

"We decided to put in place these changes to help ensure the well-being of donors," said Mark Donnison, vice-president of donor relations for Canadian Blood Services.

"The real core outcome of this is you see that very frequent donors do have a higher tendency to having lower iron stores in their bodies and it can lead to things like anemia. We've put these changes in place to help prevent that kind of occurrence."

Donnison said women traditionally have lower hemoglobin levels than men in part due to menstrual cycles.

There are hundreds of thousands of people who are relying on a very small pool of people. - Mark Donnison

"We'll often see that a female coming in is eligible one time and two or three donations later the iron level will be a little bit lower and they get deferred," he said.

"What we expect to see with this change in intervals is it will actually help improve the experience that donors are having."

The changes come at a cost.

"We've been estimating that roughly about 35,000 donations would be affected in a year," Donnison said.

"We've been very reliant on a relatively small pool of donors in the country. We're saying the changes to the hemoglobin criteria will reduce the average number of donations that somebody makes and because of that they need to get replaced with a lot more people coming forward."

Only 3.4 per cent of eligible donors in Canada give blood. About 400,000 Canadians give blood each year. The number is split 50-50 between male and female donors. Those who are older tend to donate more frequently.

Donnison said it's important to find a way to bring in a new generation of blood donors.

"There are patients right across the country every single day that need blood," he said.

"With the volume of donations that we need, with the number of patients that need transfusions ... there are hundreds of thousands of people who are relying on a very small pool of people. We need to change the dialogue and we need more people."

Donnison said Canadian Blood Services will ramp up its recruitment efforts in part by reaching out on social media.