Based on observational epidemiological research, depression following acute myocardial infarction (MI) is considered an independent risk for subsequent mortality.¹ This is despite the fact that in the only large scale randomised control trial to date, the successful treatment of depression yielded no relative advantage in terms of survival.² Furthermore, not all prospective observational studies have found an association between depression following MI and mortality.³ In such studies, depression would not appear to be related to conventional indices of disease severity. Thus, where associations between depression following MI and mortality appear, they could reflect the confounding of depression with disease severity.⁴ We have discussed elsewhere⁵ why depression and disease severity might be correlated in some studies but not in others; we have argued that it depends on the accuracy of patients’ perceptions about the severity of their condition.

Although some, but by no means all, observational studies have attempted to control statistically for disease severity, the measures employed have been varied and all are imprecise indices …