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Risk of venous thromboembolism (VTE) recurrence remains high in patients with cancer-associated thrombosis (CAT), despite therapeutic anticoagulation. Identifying patients at risk of treatment failure is still a challenge.

We aimed to assess the performance of the Ottawa score in predicting VTE recurrence in a large homogeneous population of patients with CAT treated with the same anticoagulant, tinzaparin, for at least 3 months.

Individual patient data from 3 prospective cohort studies and 1 randomized controlled trial were pooled (PROSPERO: CRD42019119907). Clinical events of interest were adjudicated by independent central adjudication committees in all 4 studies.

Among the 1413 patients included, the Ottawa score could be calculated for 1088 of whom 646 (59.4%) were classified at high risk of recurrence (Ottawa score ≥ 1). The 6-month cumulative incidence of recurrent VTE was 5.0% (95% CI, 3.2-7.8) in the Ottawa low-risk group and 8.5% (95% CI, 6.6-10.8) in the high-risk group. The area under the receiver operating characteristic curve was 0.56 (95% CI, 0.51-0.62). The sensitivity of the dichotomized Ottawa score (score ≥ 1) was 72.8% (95% CI, 62.6%-83.0%), the specificity was 41.9% (95% CI, 37.8%-45.9%), the positive predictive value was 8.6% (95% CI, 6.4%-10.8%), and the negative predictive value was 95.3% (95% CI, 93.3%-97.4%). Introducing additional predictive factors failed to significantly improve the score’s performance.

Despite the large number of patients and anticoagulant treatment standardization, the Ottawa score failed to accurately predict recurrent VTE in patients with CAT treated with tinzaparin.

• Ottawa score evaluated in a pooled cohort of 1413 patients with CAT started on tinzaparin. • Recurrent VTE rates were 5.0% in low-risk (41%) and 8.5% in high-risk (59%) groups at 6 months. • Ottawa score poorly predicted recurrent VTE (area under the curve, 0.56). • Adding other factors did not improve the score’s predictive accuracy.