The non-O ABO blood type is the most important risk factor for venous thromboembolism, making up 20% of inferred risk for the condition.

Individuals with an A or B blood type have an increased risk of venous thromboembolism or blood clots in their veins and myocardial infarction compared with individuals with O blood type.

A retrospective study was carried out by scientists at the Copenhagen University Hospital (Denmark) who looked at data on 66,001 people who had been followed for 33 years from 1977 through 2010 to determine whether ABO blood type is associated with an increased risk of venous blood clots in the general population. They determined ABO genotype from single nucleotide polymorphisms (SNP) in the ABO gene. The genotypes were validated by sequencing.

The scientists also measured the following using standard hospital assays: plasma levels of total cholesterol, low-density lipoprotein cholesterol, high density lipoprotein cholesterol, platelets, mean platelet volume, leukocytes, coagulation factors (II, VII, X), international normalized ratio, activated partial thromboplastin time, high-sensitivity C-reactive protein and complement C3.

The team found that the risk increased when ABO blood type was combined with factor V Leiden R506Q genotype or prothrombin G20210A genotype. These genetic mutations were associated with an increased risk of venous thromboembolisms. This finding confirms the conclusion of other studies. The scientists also found an 11-fold increased risk of venous thromboembolism for people with the prothrombin G20210A mutation. The population attributable risk of venous thromboembolism was 20% for ABO blood type, 10% for factor V Leiden R506Q and 1% for prothrombin G20210A.

Børge G. Nordestgaard, MD, DMSc, a coauthor of the study said, "We found an additive effect of ABO blood type on risk of venous thromboembolism when combined with factor V Leiden R506Q and prothrombin G20210A; ABO blood type was the most important risk factor for venous thromboembolism in the general population." The authors concluded that ABO blood type should be considered for inclusion in genetic screening for thrombophilia. The study was published February 4, 2013, in the Canadian Medical Association Journal.

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