Why O-negative blood is liquid gold in life-or-death situations

Without O-negative blood, many patients would die 

Information
August 27, 2021
A bag of O-negative blood at Canadian Blood Services' production site in Brampton, Ontario

A trauma patient arrives at the emergency room. Staff don’t know her blood type, and it will take about an hour to find out. She doesn’t have an hour. She’s bleeding too much.

Thankfully, she can rely on the generosity of O-negative blood donors to keep her alive when every moment matters.

O-negative blood donors, sometimes called “universal blood donors,” are essential to saving the lives of patients when we don’t know their blood type.

Triplets with O-negative blood celebrate their 17th birthday with donation

What is O-negative blood?

A person’s blood type is determined by protein and sugar molecules on their red blood cells.

“Our red blood cells have 360 protein and sugar decorations on their surfaces, called antigens, and among those 360 there are two really important antigens: ABO and Rhesus,” says Dr. Jeannie Callum, transfusion medicine specialist and hematologist at Kingston Health Sciences Centre and professor in the Department of Pathology and Molecular Medicine at Queen’s University.

The ABO system involves antigens A and B, and determines whether your blood type is A, B, AB, or O. Those with type O blood have neither antigen and comprise about 50 per cent of people worldwide.

Originally, all humans had A+ blood. For complex reasons, lacking the A and B antigens having the A antigen reduces the chance of death from malaria. So, evolution has driven a gradual increase in the number of people with type O blood over the course of many millennia. In parts of sub-Saharan Africa where malaria is most prevalent, 80 per cent of people have type O blood.

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The other most important system, the Rhesus or Rh system, involves a protein called the D antigen. If your red blood cells have the D antigen, you have a positive blood type. If they don’t, your blood type is negative. Most people in Canada are positive for the D antigen — about 85% — and in African and Asian nations that percentage is even higher.

Why blood type matters

Our immune systems are designed to know what belongs in our body and what doesn’t. When recipients get blood containing antigens that don’t occur in their own blood, things can go very wrong.

“If you don’t have an antigen, your immune system is already primed to the antigen, and someone puts blood containing that antigen into you, your immune system goes into attack mode and blows up those transfused blood cells. Your kidneys are likely to go into failure, you get really sick, and many patients in that situation will die,” says Dr. Callum.

The D-antigen is also very important because if a person with child-bearing potential is Rh-negative and receives Rh-positive blood, they will likely develop an antibody that could cause future pregnancy complications and losses.

Because O-negative blood lacks the A, B and D antigens it is safe for patients when we don’t know their blood type. It’s also the only blood that’s safe for O-negative recipients.

“Every time a person is in a trauma or losing a lot of blood, we’re going to use O-negative blood till we know their blood type. We will switch when we know it’s safe, but it takes about an hour to find out and that window is critical,” says Dr. Callum.

Dr Callum donating o-negative blood.


Dr. Jeannie Callum donated blood at an event in Toronto in mid-March 2020 (prior to the implementation of Canadian Blood Services mask requirements to prevent the spread of COVID-19).

Dr. Callum is an O-negative blood, platelet and stem cell donor herself.

“It’s a little magical to donate blood and know that about a week later your blood is in a patient somewhere and helping them feel better. I feel it’s my duty to donate while I’m healthy and able,” she says. “Especially because I know a lot of O-negative blood is used for emergency resuscitation and newborn babies."

Learn more about blood type, including O-negative blood

How we help patients with rare blood

Do you know your blood type? Only seven per cent of Canadians have universal O-negative blood, and you could be one of them. Find out if you have type O-negative blood by booking an appointment to donate at blood.ca.

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