A study was conducted to identify factors associated with rebleeding in patients admitted to the hospital with upper gastrointestinal hemorrhage. The medical charts of 2,488 patients were analyzed retrospectively, and a logistic regression analysis was conducted to identify potential risk factors for rebleeding. An excerpt of study results is shown below. No missing data points regarding the type of endoscopic therapy received, comorbidities, or Heliobacter pylori status were reported in the study. Which of the following is the best interpretation of the study results?

Question

A study was conducted to identify factors associated with rebleeding in patients admitted to the hospital with upper gastrointestinal hemorrhage.  The medical charts of 2,488 patients were analyzed retrospectively, and a logistic regression analysis was conducted to identify potential risk factors for rebleeding.  An excerpt of study results is shown below.  No missing data points regarding the type of endoscopic therapy received, comorbidities, or Heliobacter pylori status were reported in the study.  Which of the following is the best interpretation of the study results?

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Explanation: The odds ratio (OR) is a measure of association between an exposure and an outcome (eg, disease). It represents the odds that an outcome will occur in the presence of an exposure divided by the odds that the outcome will occur in the absence of that exposure. An OR should be evaluated as follows: OR < 1 means that the exposure is associated with lower odds of the outcome compared to no exposure OR = 1 means that the exposure is not associated with the outcome OR > 1 means that the exposure is associated with higher odds of the outcome compared to no exposure The confidence interval (CI) of an OR indicates, with a certain confidence level, whether a given OR is statistically different from the null value (ie, OR = 1.0). A CI that excludes the null value is statistically significant (at that confidence level). In this study, positive Helicobacter pylori infection has an OR = 0.351. Because OR < 1.0, the odds of rebleeding among patients who are H pylori-positive is lower than the odds of rebleeding among patients who are H pylori-negative. The change (OR = 0.351 compared to OR = 1.0) represents a relative decrease of (1.0 − 0.351) / 1.0 = 0.649, or about 65%. Therefore, the odds of rebleeding are about 65% lower among patients who are H pyloripositive compared to patients who are H pylori negative. The OR's 95% confidence level (CI: 0.271-0.910) excludes the null value (OR = 1.0), so the association is statistically significant. (Choices A and E) Endoscopic therapy with epinephrine, thermal coagulation, or a combination of both is associated with higher odds of rebleeding (with combination therapy being associated with lower odds of rebleeding than either epinephrine or thermal coagulation alone), not with higher odds of controlling bleeding. Regardless, statistical association does not imply causation, and the differences in odds of rebleeding may be related to the underlying pathology rather than the endoscopic intervention itself (eg, higher-risk lesions may benefit from different endoscopic therapies and be associated with different odds of rebleeding). (Choice B) An OR = 2.5 does not reflect the average number of comorbidities; instead, it means that a higher number of comorbidities is associated with higher odds of rebleeding. (Choice C) The study provides information on sex as a potential risk factor for rebleeding but does not provide information on sex as a potential risk factor for hematochezia. Educational objective: An odds ratio (OR) >1 means that the exposure is associated with higher odds of the outcome. An OR <1 means that the exposure is associated with lower odds of the outcome. References: Explaining odds ratios. (http://www.ncbi.nlm.nih.gov/pubmed/20842279) Statistics notes. The odds ratio. (http://www.ncbi.nlm.nih.gov/pubmed/10827061)

A Study Was Conducted to Identify Factors Associated with Rebleeding