Hospitalization as a trigger for venous thromboembolism - Results from a population-based case-crossover study

Background: Previous studies have reported that around 50% of patients with venous thromboembolism (VTE) has undergone recent hospitalization. However, studies on the impact of hospitalization as a trigger factor for VTE are limited. Objectives: To investigate the impact of hospitalization with and without concurrent immobilization as a trigger factor for VTE. Methods: We conducted a case-crossover study of 530 cancer-free VTE patients. Hospitalizations were registered during the 90-day period preceding the VTE diagnosis (hazard period), and in four preceding 90-day control periods. A 90-day washout period between the control- and hazard periods was implemented to avoid potential carry-over effects. Conditional logistic regression was used to calculate odds ratios (OR) of VTE according to hospitalization. Results: In total, 159 (30%) of the VTE-patients had been hospitalized in the hazard period, and the OR of hospitalization was 9.4 (95% CI: 6.8-12.8). The risk increased slightly with the total number of days spent in hospital (OR per day: 1.11, 95% CI: 1.04-1.18), and with the number of hospitalizations (OR 8.9, 95% CI: 6.4-12.4 for 1 hospitalization and OR 12.3, 95% CI 6.4-23.6 for >= 2 hospitalizations). Hospitalization without immobilization was 6-times (OR: 6.3, 95% CI: 4.4-9.2) more common, whereas hospitalization with immobilization was near 20-times (OR: 19.8, 95% CI: 11.5-34.0) more common in the 90-days prior to a VTE compared to the control periods. Conclusions: Hospitalization is a major trigger factor for VTE also in the absence of immobilization. However, immobilization contributes substantially to the risk of VTE among hospitalized patients.