Mild antithrombin deficiency and risk of recurrent venous thromboembolism: a prospective cohort study.

BACKGROUND: Antithrombin deficiency, defined by antithrombin levels of <70%, is a major thrombophilic condition associated with an increased risk of venous thromboembolism (VTE). No prospective data are available about the risk of recurrent VTE associated with mildly decreased antithrombin levels (70-80%). METHODS AND
RESULTS: Consecutive patients with a first VTE were stratified according to functional antithrombin levels (<70%, 70-80%, >80%) and were followed up for a mean of 8.70 years to assess the incidence of VTE recurrence. A total of 823 patients (mean age, 48.3 years; 41.9% male) were enrolled. Recurrent VTE occurred in 253 patients (3.53% per patient-year). With stratification for antithrombin levels, VTE recurrence occurred in 19 patients with antithrombin levels <70% (5.90% per patient-year), in 20 patients with antithrombin levels 70% to 80% (5.35% per patient-year), and in 214 patients with antithrombin levels >80% (3.31% per patient-year). After adjustment for major VTE risk factors and for anticoagulation duration, the risk of VTE recurrence was significantly higher in patients with antithrombin levels <70% (hazard ratio, 3.48; 95% confidence interval, 2.16-5.61) and antithrombin levels 70% to 80% (hazard ratio, 2.40; 95% confidence interval, 1.51-3.80) compared with patients with antithrombin levels >80%. When the population was stratified according to the presence or absence of major risk factors for the index event, the association remained significant only in patients with unprovoked VTE.
CONCLUSIONS: The presence of mild antithrombin deficiency (70-80% antithrombin) in patients with unprovoked VTE is associated with a significantly increased risk of recurrence and should be taken into account when the duration of secondary prevention is determined. CLINICAL TRIAL REGISTRATION URL: http://www.clinicaltrials.gov. Unique identifier: NCT01382550.