LETTER TO THE EDITOR
Year : 2015 | Volume
: 59 | Issue : 1 | Page : 70-
Are confounders important in a study?
Neha Singh1, Taramangalam Ramakrishnan2, Anurag Khera2,
1 Resident, Department of Community Medicine, Armed Forces Medical College (AFMC), Pune, Maharashtra, India
2 Assistant Professor, Department of Community Medicine, Armed Forces Medical College (AFMC), Pune, Maharashtra, India
Resident, Department of Community Medicine, Armed Forces Medical College (AFMC), Pune - 411 040, Maharashtra
|How to cite this article:|
Singh N, Ramakrishnan T, Khera A. Are confounders important in a study?.Indian J Public Health 2015;59:70-70
|How to cite this URL:|
Singh N, Ramakrishnan T, Khera A. Are confounders important in a study?. Indian J Public Health [serial online] 2015 [cited 2023 Feb 1 ];59:70-70
Available from: https://www.ijph.in/text.asp?2015/59/1/70/152880
We read with great interest the article titled "A case control study examining association between infectious agents and acute myocardial infarction" by Shrikhande, Zodpey and Nagandhi.  We would like to congratulate the authors for bringing to light a new area of research regarding association between infectious agents and incidence of myocardial infarction (MI). However, we would like to draw attention to a few observations that we came across in the article.
We find that it would be more appropriate to name the study design as a cross-sectional analytical study rather than a case control study as the data for exposure and outcome among the cases and controls was collected at the same point of time as is done in a cross-sectional study, instead of taking a retrograde history of exposure among cases and controls as is the protocol in a case control study. 
The outcome of the study is taken as incident cases of MI and the sampling frame for the cases is formed by the patients admitted in the ICU and cardiology ward of the hospital. However, it is known that 30% of deaths occur within first hour of onset of MI;  hence, the sampling frame excludes a major proportion of patients with incident MI which may be different from the sampled cases. This may be mentioned as a limitation of the study.
The inclusion/exclusion criteria for controls in the study are not clearly defined as the morbidity of the controls may affect the results of the study. For example, if the controls are patients from a gastrointestinal ward (cases of peptic ulcer, gastritis, etc), the results of the study may favor null hypothesis. Similarly, if controls are included from a medical ward with a preponderance of patients with diabetes mellitus and hypertension who are instructed to follow dietary restrictions and smoking (independent risk factors for C. pneumoniae),  selection of such controls may exaggerate the results of the study.
The causation of MI is known to be influenced by multiple risk factors,  the common risk factors being family history, body mass index, level of physical activity, and diet. A stratified analysis of results considering the confounders would have been more informative and appropriate.
Infection with C. pneumonia is known to be higher among smokers as compared to the general population.  Also, there is evidence suggesting higher incidence of MI among smokers. Hence, a reverse causation pathway for the incidence of MI is possible among the cases of the study, which if not addressed would exaggerate the results of the study.
|1||Shrikhande SN, Zodpey SP, Negandhi H. A case-control study examining association between infectious agents and acute myocardial infarction. 2014;58:106-9.|
|2||Hennekens CH, Buring JE. Epidemiology in Medicine. New York: Lippincott Williams & Wilkins; 1987. p. 102-12.|
|3||Longo DL, Fauci AS, Kasper DL, Hauser SL, Jameson JL, J. Harrison's Principles of Internal Medicine. 18 [th] ed. USA: McGraw-Hill Professional; 2011. p. 2021.|
|4||Hahn DL, Golubjatnikov R. Smoking is a potential confounder of the Chlamydia pneumoniae-coronary artery disease association. Arterioscler Thromb 1992;12:945-7.|
|5||Patel P, Mendall MA, Carrington D, Strachan DP, Leatham E, Molineaux N, et al. Association of Helicobacter pylori and Chlamydia pneumoniae infections with coronary heart disease and cardiovascular risk factors. 1995;311:711-4.|