Summary
Background Deep venous thrombosis is a common disease, with genetic and acquired risk
factors. Many patients have a history of minor events (short periods of immobilisation
such as prolonged travel, short illness, minor surgery or injuries) before onset of
venous thrombosis. However, the role of these minor events has received little formal
study. Also, we do not know how minor events might interact with the presence of genetic
prothrombotic defects (factor V Leiden mutation, factor II mutation, protein C, S
and antithrombin deficiency). Patients and Methods On the basis of case-control data
from a thrombosis service in the Netherlands, we added a follow-up period for a casecross-over
analysis of minor events as risk factors, and a case-only analysis for the interaction
with factor V Leiden. A total of 187 patients with first, objectively diagnosed venous
thrombosis of the legs, aged 15–70, without underlying malignancies and without major
acquired risk factors entered the study. For the analysis of minor events in the case-cross-over
analysis, we used a matched odds ratio; in the caseonly analysis, we used the multiplicative
synergy index. Results In 32.6% of the 187 patients with deep venous thrombosis who
did not have major acquired risk factors, minor events were the only external risk
factors. Minor events increased the risk of thrombosis about 3-fold, as estimated
in the case-cross-over analysis (odds ratio 2.9, 95% confidence interval 1.5–5.4).
The synergy index between minor events and factor V Leiden mutation in the case-only
analysis was 0.7 (95% confidence interval 0.3–1.5). Therefore, persons with factor
V Leiden mutation who experience a minor event will have an estimated risk increase
of about 17-fold, which exceeds the sum of the individual risk factors. Conclusions
Minor events are likely to play an important role in the development of deep venous
thrombosis, especially in the presence of genetic prothrombotic conditions.
Keywords
Venous thromboembolism - epidemiology - travel - immobilisation - risk factors - factor
V
