Objective: To assess the presence of publication bias and its relation to geographical bias in clinical trials involving complementary and alternative medicine (CAM) published in the highest impact factor general medicine journals.
Study design and setting: All CAM clinical trials published in the four highest impact factor general medicine journals, Lancet and British Medical Journal (European), and New England Journal of Medicine and Journal of American Medical Association (U.S.), between 1965 and 2004 were abstracted using Medline. Three reviewers abstracted data from the individual studies. In a multivariate analysis, factors predictive of a positive study were assessed.
Results: A total of 259 studies met the inclusion criteria. CAM trials published in the European journals were significantly more likely to be positive compared to those published in the U.S. journals (76% vs. 50%, odds ratio [OR]=3.15, P<0.0001). Studies originating outside of the United States were significantly more likely to be positive compared to the U.S. studies (75% vs. 49%, P<0.0001). Adjusting for location and other variables in a multivariate model, the OR for European vs. U.S. journals to publish a positive CAM trial was 1.95 (P=0.11).
Conclusion: Publication bias related to CAM trials among the highest impact factor general medicine journals is partly due to geographical bias.