Chronic obstructive pulmonary disease as a cardiovascular risk factor: results of a case-control study (CONSISTE study)

To the editor

I read with interest the article “Chronic obstructive pulmonary disease as a cardiovascular risk factor. Results of a case-control study (CONSISTE study)” by de Lucas-Ramos et al.¹ In my opinion, the study did not use case-control design, despite its title.

The study participants were not selected on the basis of the outcome of interest, ie, cardiovascular disease (CVD), as is done in typical case-control studies. Rather, the study participants were selected according to the main exposure of interest which is presence or absence of diagnosis of chronic obstructive pulmonary disease with the CVD outcome of interest measured at a later point in time, thereby making it a retrospective cohort study. The authors even went further to state that “all patients had had a previous follow-up of more than 1 year.”¹ Follow-up time is a feature of cohort study and neither case-control nor cross-sectional study.

Correspondence: Pilar de Lucas Ramos, Pneumology Department, Hospital General Universitario, Gregorio Marañon, C/Dr Esquerdo 46, 28007 Madrid, Spain, Email [email protected]

We read with interest the letter sent by Dr Michael Falola related to our study “Chronic obstructive pulmonary disease as a cardiovascular risk factor. Results of a case-control study (CONSISTE study)”¹ and would like to answer that letter.

Although defining “case” as a patient with chronic obstructive pulmonary disease (COPD) and “control” as a patient without COPD, the study would seem to be directed to establish if ischemic heart disease is a risk factor for COPD, instead of if COPD is a risk factor for ischemic heart disease. In the logistic regression analysis, the ischemic heart disease was considered as the dependent variable while COPD was treated as a predictor factor, so it is correct to denominate the study as a case and controls one. With regard to the second problem, regardless of whether there was a previous follow-up period, otherwise necessary to ensure the diagnosis of COPD, all variables were taken simultaneously in a precise time, and therefore it is a cross-sectional study.

We consider therefore that the study methodology is correct and allows us to set the conclusions.