Does Blood Group Affect Risk of Bleeding Events or Bleeding Severity?

According to research published in Blood Advances, people who experience bleeding of unknown causes are 48% more likely to have blood group O, compared with patients without unexplained bleeding. Study authors, led by Dino Mehic, MD, PhD-c, from the Medical University of Vienna in Austria, also found that patients with blood group O have more severe bleeding symptoms than those with other blood types.

“Blood group O is associated with lower von Willebrand factor (VWF) levels and has been found to be overrepresented in patients with von Willebrand disease type I,” Dr. Mehic and coauthors wrote. “Still, the well-known association of blood types other than O and risk of thrombosis is independent of VWF and factor VIII (FVIII) [and] the role of blood group O as an independent risk factor for bleeding remains unclear.”

To identify novel mechanisms that influence bleeding tendency in patients with bleeding of unknown cause (i.e., those without abnormalities in plasmatic coagulation or platelet function), the investigators analyzed bleeding rates and bleeding severity among patients enrolled in the Vienna Bleeding Biobank cohort.

Patients with blood group O had more severe bleeding and more bleeding symptoms, after adjustment for von Willebrand factor levels, factor VIII levels, and sex.

Researchers found 422 patients with bleeding of unknown cause and no previous diagnosis of an established bleeding disorder. The patients’ blood groups were as follows:

  • O: 199 (47.2%)
  • A: 149 (35.3%)
  • B: 49 (11.6%)
  • AB: 25 (5.9%)

They then compared the prevalence of blood group O in patients with bleeding of unknown cause with its prevalence in a group of 23,145 healthy blood donors. In the healthy blood donor group, 37.6% of people had blood group O, confirming that patients who experience bleeding are more likely to have blood group O (odds ratio = 1.48; p<0.001).

The authors added that blood group A was underrepresented among patients with bleeding events compared with the healthy blood donor cohort (35.3% vs. 42.1%), while there were no substantial differences in the occurrence of blood groups B and AB.

When Dr. Mehic and coauthors analyzed bleeding severity according to the blood groups, they determined that bleeding was more severe (measured by Vicenza bleeding score) among people with blood group O. Blood group O patients also had more bleeding symptoms – an association that persisted after adjustment for VWF levels, FVIII levels, and sex (TABLE). For example, oral mucosal bleeding occurred in 26.1% of patients with blood group O versus 14.3% of those with non-O blood group.

Next, researchers sought to identify potential mechanisms underlying the effect of blood group O on bleeding phenotype by analyzing results from global hemostatic and platelet function tests.

There was no difference in thrombin generation, clot lysis, or platelet function when comparing patients with unexplained bleeding with blood group O to those with non-O. Those with blood group O had increased clot density. Again, these associations were independent of VWF and FVIII, which have often been presented as possible causes of increased bleeding tendency in patients with blood group O.

The authors noted that the lack of an explanation for the increased bleeding severity in patients with blood group O represents a limitation of the study. “However, we are convinced that our data encourage studies with the purpose of further elucidating the mechanisms behind these observations,” they wrote.

The authors report no relevant conflicts of interest.


Mehic D, Hofer S, Jungbauer C, et al. Association of ABO blood group with bleeding severity in patients with bleeding of unknown cause. Blood Adv. 2020;4(20):5157-5164.