ABO blood group type and risk of venous thromboembolism in patients with cancer.

Venous thromboembolism (VTE) is common in cancer patients. While in the general population blood type non-O is associated with increased VTE risk, the impact of ABO blood type on risk of cancer-associated VTE has not been clarified. To determine the influence of ABO blood type on cancer-associated VTE risk, we conducted an analysis within the Vienna Cancer and Thrombosis Study (CATS), a prospective cohort study including patients with newly diagnosed or recurrent cancer observed for the primary outcome VTE. Restricted cubic spline analysis was performed and specific time-restricted subdistribution hazard ratios (SHR) were calculated to investigate the association between non-O blood type and VTE over time. 1,708 patients were included in the analysis (median follow-up time: 24 months; interquartile range: 10-24), and 151 patients developed VTE (8.8%). During the first 3 months of follow-up, there was no association between non-O blood type and VTE risk (SHR: 1.00, 95% CI: 0.60-1.67). Thereafter, non-O blood type was associated with a higher VTE risk (SHR 1.79, 95%CI: 1.12-2.85). Furthermore, non-O blood type was associated with increased VTE risk in patients with intermediate and low thrombotic risk tumor types (SHR 1.73, 95% CI: 1.09-2.73), but not in very high-risk types (pancreatic, gastroesophageal and brain cancer; SHR 0.94, 95% CI: 0.55-1.61). This association was weakened after adjustment for factor VIII. Non-O blood type is a time-dependent predictor of VTE in cancer patients. It is associated with increased VTE risk beyond 3 months of follow-up and in patients with intermediate and low-risk tumor types.