BACKGROUND: The magnitude of the association\ud between Helicobacter pylori and\ud incidence of gastric cancer is unclear. H\ud pylori infection and the circulating antibody\ud response can be lost with development\ud of cancer; thus retrospective studies\ud are subject to bias resulting from classifi-\ud cation of cases as H pylori negative when\ud they were infected in the past.\ud AIMS: To combine data from all case control\ud studies nested within prospective\ud cohorts to assess more reliably the relative\ud risk of gastric cancer associated with H\ud pylori infection.To investigate variation in\ud relative risk by age, sex, cancer type and\ud subsite, and interval between blood sampling\ud and cancer diagnosis.\ud METHODS: Studies were eligible if blood\ud samples for H pylori serology were collected\ud before diagnosis of gastric cancer in\ud cases. Identified published studies and two\ud unpublished studies were included. Individual\ud subject data were obtained for\ud each. Matched odds ratios (ORs) and 95%\ud confidence intervals (95% CI) were calculated\ud for the association between H pylori\ud and gastric cancer.\ud RESULTS: Twelve studies with 1228 gastric\ud cancer cases were considered. The association\ud with H pylori was restricted to noncardia\ud cancers (OR 3.0; 95% CI 2.3–3.8)\ud and was stronger when blood samples for\ud H pylori serology were collected 10+ years\ud before cancer diagnosis (5.9; 3.4–10.3). H\ud pylori infection was not associated with an\ud altered overall risk of cardia cancer (1.0;\ud 0.7–1.4).\ud CONCLUSIONS: These results suggest that\ud 5.9 is the best estimate of the relative risk\ud of non-cardia cancer associated with H\ud pylori infection and that H pylori does not\ud increase the risk of cardia cancer. They\ud also support the idea that when H pylori\ud status is assessed close to cancer diagnosis,\ud the magnitude of the non-cardia\ud association may be underestimated
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