BACKGROUND: Venous stasis, which can occur with prolonged sedentary behavior (SB), is associated with venous thromboembolism (VTE) risk, but VTE risk associated with accelerometer-measured SB has not been quantified. OBJECTIVES: To evaluate accelerometer-based measures of SB in relation to incident VTE. METHODS: We included 5,591 participants, aged 63-99 years, of the Women's Health Initiative Objective Physical Activity and Cardiovascular Health cohort study without prior VTE. Between May 2012-2014, participants wore the ActiGraph GT3X+ accelerometer at the hip for 7 days. Three SB measures were classified using the Convolutional Neural Network Hip Accelerometer Posture algorithm: total sitting time, mean sitting bout duration, and total time spent in prolonged (≥30 minute) sitting bouts. VTE events were centrally adjudicated. Multivariable-adjusted Cox models estimated hazard ratios (HRs) for each SB and VTE risk. Women were censored at first VTE, death, loss to follow-up, or February 2023. Mediation by body mass index (BMI) was evaluated. RESULTS: Over a mean follow-up of 8.2 years, 229 women experienced a VTE. In adjusted models, longer mean sitting bout duration was associated with greater incident VTE risk (HR per 5-minute increase=1.15
95% CI: 1.04, 1.28). BMI mediated approximately 30% of this association (p<
0.01). We found no significant evidence that total sitting time or total time spent in prolonged sitting bouts were associated with VTE. CONCLUSION: Longer mean sitting bout duration was associated with greater VTE risk, with substantial mediation by BMI. Behavioral efforts to reduce sedentary bout length in older women may reduce their VTE risk.