Analysing case-control studies: adjusting for confoundingBMJ 2013; 346 doi: http://dx.doi.org/10.1136/bmj.f25 (Published 04 January 2013) Cite this as: BMJ 2013;346:f25
- Philip Sedgwick, reader in medical statistics and medical education
- 1Centre for Medical and Healthcare Education, St George’s, University of London, Tooting, London, UK
A case-control study examined the association between chronic Helicobacter pylori infection and coronary heart disease at young ages. In total, 1122 survivors of suspected acute myocardial infarction at age 30-49 years were recruited. For each case, a control matched for age and sex with no history of coronary heart disease was enrolled. Chronic infection with H pylori was confirmed serologically. Information on other risk factors for coronary heart disease was collected, including smoking behaviour, indicators of socioeconomic status, obesity, and blood lipid concentrations. Controls were asked about their current habits and history, whereas cases were asked about their habits and history just before their index myocardial infarction. Blood samples were obtained from cases within 24 hours of the onset of symptoms and from controls after collection of the information about risk factors.1
Early onset myocardial infarction was significantly associated with seropositive H pylori infection antibodies (odds ratio 2.28 (99% confidence interval 1.8 to 2.9)). The odds ratio was reduced to 1.87 (1.42 to 2.47) after adjustment for smoking and indicators of socioeconomic status and to 1.75 (1.29 to 2.36) after additional adjustment for blood lipid concentrations and obesity. Therefore, a moderate association existed between coronary heart disease and H pylori infection seropositivity that could not be fully explained by other risk factors.
Which of the following statements, if any, are true?
a) Matching ensured that any differences between cases and controls were not due to differences in age and sex.
b) The case-control study estimated the population at risk.
c) The adjusted odds ratios could have been derived using logistic regression.
d) The association between H pylori seropositivity and coronary heart disease was independent of smoking and indicators of socioeconomic status.
Statements a, c, and d are true, whereas b is false.
The purpose of the case-control …