A new study, carried out in Japan, finds that people who have type O blood are more likely to die following severe trauma.

Everyone reading this article will fit into one of four blood groups: A, B, AB, or O.

This is determined by the genes that you have inherited from your parents.

Before diving into the study, it's worth briefly reminding ourselves what each blood group means.

Blood groups take their names from the type of antigen on the surface of red blood cells and the type of antibodies in the plasma.

Blood group A has A antigens on red blood cells and anti-B antibodies in the plasma, while blood group B has B antigens and anti-A antibodies in the plasma. AB possesses both antigens but no antibodies, and O has no antigens on the red blood cell surface but A and B antibodies in the plasma.

Type O is the most common blood group, accounting for around half of all people in the United States. Because of its prevalence, hospital stocks often run worryingly low.

The trouble with type O

A new study—carried out at Tokyo Medical and Dental University Hospital in Japan—investigated whether an individual's blood group affected their risk of death following severe trauma. This is an injury with the potential to cause death or long-term disability.

Study coauthor, Dr. Wataru Takayama, explains why he investigated this, saying, "Recent studies suggest that blood type O could be a potential risk factor for hemorrhage (bleeding in large quantities)."

"Loss of blood," he adds, "is the leading cause of death in patients with severe trauma, but studies on the association between different blood types and the risk of trauma death have been scarce."

Dr. Takayama goes on, "We wanted to test the hypothesis that trauma survival is affected by differences in blood types."

To investigate, the researchers dug into data taken from 901 patients with severe trauma from two emergency care centers in Japan between 2013 and 2016.

They compared the death rates of patients with type O blood with those who were not type O, and their findings are published in the journal Critical Care.

Patients who were not blood type O had an 11% death rate. However, those who were blood type O had a death rate of 28%—almost three times higher.

Why is the risk higher for type O?

The scientists believe that this stark difference in death rates may be due to a blood clotting agent known as von Willebrand factor. This is found in lower levels in people with type O blood, perhaps increasing the risk of hemorrhage.

These findings back up earlier studies. For instance, one study found that venous thrombosis—a blood clot in a vein—is more likely to occur in patients who are not type O.

Similarly, another found that being blood group O increased the risk of postpartum hemorrhages, or significant blood loss following childbirth.

More research is sorely needed; if type O blood behaves differently, there may need to be a change in the way that it is handled.

"Our results also raise questions about how emergency transfusion of O type red blood cells to a severe trauma patient could affect homeostasis, the process which causes bleeding to stop, and if this is different from other blood types."

–Dr. Wataru Takayama

The research does have some limitations, as the study authors explain. For instance, in this study, all patients were Japanese, so the study would need to be replicated in people from other ethnic backgrounds.

Also, the analysis compared patients with type O blood with patients without type O blood, and they did not break the differences down into the other three groups. This means that other differences might be buried in the data.

Dr. Takayama plans to continue his research in this area. "Further research," he says, "is necessary to investigate the results of our study and develop the best treatment strategy for severe trauma patients."