The contemporary natural history of patients with acute pulmonary embolism (PE) not receiving (or early discontinuing) anticoagulant therapy has not been consistently evaluated.

To assess the rate of the composite outcome of PE-related death, sudden death, or recurrent thromboembolism (VTE) within 30 days in all PE patients in whom anticoagulation was not administered or discontinued prematurely (<90 days of anticoagulation).

We used the RIETE database to assess the incidence rates (per 100 person-days) of the composite outcome within the subsequent 30 days. The risk of these events was compared to PE patients who were anticoagulated for ≥90 days.

Of 34,447 PE recruited from 2001 to 2017, 47 (0.14%) did not receive anticoagulants and 1348 (3.91%) discontinued it before 90 days. Fatal PE developed in 25 (53%) of those without any anticoagulation and in 45 (3.33%) with premature discontinuations. Sudden death or non-fatal recurrent VTE occurred in 6 (0.45%) and 24 (1.48%) patients, respectively. The incidence of the primary outcome declined logarithmically from 6.36 per 100 patient-days in untreated patients to 0.32-0.13 in those treated for 8-90 days. During the first week of follow-up, the incidence rate was 13.9 and 0.60-0.31 per 100 patient-days, respectively. The adjusted odds of the primary outcome was 27 fold higher in untreated than in treated patients, and progressively decreased to 2.5-7 fold higher in patients treated for at least 7 days.

The incidence of the composite outcome was highest during the first week, and inversely and logarithmically correlated with the duration of anticoagulant therapy.