Air Travel and Venous Thrombosis: How Much Help Might Aspirin Be?

Yoon K Loke, MBBS; Sheena Derry, MA

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Methods

We have used a measure of treatment benefit, the number-needed-to-treat (NNT), which is easily understood by both patient and physician. We have calculated the NNT for aspirin to prevent 1 DVT in travelers undertaking long journeys, using published estimates of (1) the rate of DVTs arising from long-distance travel, and (2) the relative efficacy of aspirin in preventing DVT.

Although no definitive studies are available yet, experts on the British Parliamentary Select Committee have estimated the possible increased incidence of DVT due to long-distance travel to be 0-40 per 100,000 travelers making 1 long-distance journey per year (with 20 per 100,000 being cited as the most likely figure).[1] In view of the uncertainties surrounding this estimate, and the widespread variation in risk among individual travelers, we decided to calculate NNTs across the range of risks.

We performed searches of MEDLINE and EMBASE databases from 1980-2001 using the following strategy -- aspirin and (vein or venous) and thromb* -- to retrieve evidence on the effectiveness of aspirin in preventing DVT. There were no trials in long-distance travelers, although aspirin has been used for DVT prophylaxis in other areas. Two meta-analyses in 1994, based mainly on small trials, did not show statistically significant effects for aspirin used alone.[2,3] However, a recent large, randomized, controlled trial of aspirin against placebo in 13,000 patients with hip fractures found a 29% relative risk reduction in DVT with 160 mg of aspirin taken daily for 5 weeks.[4] Although there are obvious problems with extrapolating the data to long-distance travelers, this is the best evidence we could find to justify aspirin use.

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