Inflammatory bowel disease and the risk of venous thromboembolism

I never really understood why IBD was such a high risk condition. Apparently it was known to be so because the Chest guidelines gave it special mention in their recommendations for VTE prophylaxis in medical patients.

Now a new Lancet paper reports this:

13756 patients with inflammatory bowel disease and 71672 matched controls were included in the analysis, and of these 139 patients and 165 controls developed venous thromboembolism. Overall, patients with inflammatory bowel disease had a higher risk of venous thromboembolism than did controls (hazard ratio 3·4, 95% CI 2·7–4·3; p less than 0·0001; absolute risk 2·6 per 1000 per person-years). At the time of a flare, however, this increase in risk was much more prominent (8·4, 5·5–12·8; p less than 0·0001; 9·0 per 1000 person-years). This relative risk at the time of a flare was higher during non-hospitalised periods (15·8, 9·8–25·5; p less than 0·0001; 6·4 per 1000 person-years) than during hospitalised periods (3·2, 1·7–6·3; p=0·0006; 37·5 per 1000 person-years).

So let's see if I understand this. IBD in general carries an increased risk of VTE. That risk is even higher during an active flare. But why was the difference between patients with and without a flare less in hospitalized patients? Perhaps because other VTE risk factors inherent in just being hospitalized may dilute out the difference.

IBD must be an under-appreciated VTE risk because it isn't talked about all that much. It deserves inclusion on the list of usual suspects such as hereditary thrombophilia, cancer, severe acute respiratory disease, etc. I also have to wonder how many patients are deprived of chemical prophylaxis because of a perceived risk of GI bleeding.

H/T to Clinical Cases and Images.