Meta-analysis: Travel associated with threefold increase in risk for VTE

Travel was associated with a threefold increased risk for venous thromboembolism, according to data from a meta-analysis. Additionally, each two-hour increase in travel duration resulted in an 18% increased risk for VTE.

The analysis included 14 studies that examined the association between travel via any mode of transportation and VTE; all studies included nontraveling controls. The researchers extracted data on study and patient characteristics, risk estimates and quality parameters. The purpose of the analysis was to estimate the risk for VTE in travelers, determine the existence of a dose-response relationship and identify reasons for contradictory results of previous studies.

The abstracts used for the analysis included 4,055 cases of VTE. The overall pooled RR for VTE in travelers was 2.0 (95% CI, 1.5-2.7) compared with nontravelers. Heterogeneity existed (P=.008) whether the studies used referred or nonreferred controls.

When studies using referred controls were excluded, the pooled RR for VTE in travelers was 2.8 (95% CI, 2.2-3.7), without significant heterogeneity.

To reduce selection bias, the researchers excluded case-control studies that used referred controls from their final analysis. They stated that, “in any study design, persons who do not experience a disease should never be expected to be similar to the case-patients; those who develop a disease will always have many more risk factors for disease than those who do not develop the disease.”

According to an accompanying editorial by Jan P. Vandenbroucke, MD, PhD, Leiden University Medical Center, and colleagues, the researchers’ exclusion was probably appropriate, but for more complex reasons.

“Like any choice of controls, the use of referred controls in case-controlled studies can be a double-edged sword: Although it can mislead, it also has the power to resolve a controversy when a postulated diagnostic referral bias does not exist or is minimal, as was the case for oral contraceptive and VT,” they wrote.

There was a dose-response relationship that yielded an 18% higher risk for VTE for each two-hour increase in duration of travel by any mode (P=.010) and a 26% higher risk for every two hours of air travel (P=.005), according to the researchers.

Chandra D. Ann Intern Med. 2009;151:1-11.