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Division of Clinical Epidemiology, Royal Victoria Hospital, McGill University Health Centre, and the Depts of Epidemiology and Biostatistics and of Medicine, McGill University, Montreal, Canada
CORRESPONDENCE: S. Suissa, Division of Clinical Epidemiology, Royal Victoria Hospital, 687 Pine Avenue West, Ross 4.29, Montreal, Canada, H3A 1A1. Fax: 1 5148431493. E-mail: samy.suissa@clinepi.mcgill.ca
Keywords: biases, cohort studies, drug effectiveness, epidemiology, immortal time, pharmacoepidemiology
Received: June 3, 2003
Accepted October 5, 2003
This research was funded by grants from the Canadian Institutes of Health Research (CIHR) and Fonds de la recherche en Santé du Québec (FRSQ). The acquisition of the database was funded by grants from AstraZeneca, Boehringer Ingelheim and GlaxoSmithKline. The author is the recipient of a Distinguished Investigator award from the CIHR. This study is based on de-identified data provided by the Saskatchewan Dept of Health. The interpretation and conclusions contained herein do not necessarily represent those of the government of Saskatchewan or the Saskatchewan Dept of Health.
A recent observational study, which suggested that inhaled corticosteroids (ICS) with or without long-acting bronchodilators are effective at reducing all-cause mortality in chronic obstructive pulmonary disease (COPD) patients, may be subject to immortal time bias.
This bias was assessed using a population-based cohort of 3,524 newly treated COPD patients from Saskatchewan, Canada, observed from 1990–1999. Regular users of bronchodilators or ICS were followed for 3 yrs, during which time 860 deaths occurred. Cox's proportional hazards model was used to compare the hierarchical intention-to-treat approach employed in the recent study, a technique subject to bias from two sources of immortal time, with the conventional intention-to-treat approach and the according-to-treatment approach.
The adjusted rate ratio of death using the hierarchical intention-to-treat approach was 0.66 (95% confidence interval (CI) 0.57–0.76) for ICS use relative to bronchodilator use, compared with 0.75 (95% CI 0.62–0.90) with the conventional intention-to-treat approach. Conversely, the rate ratio was 0.94 (95% CI 0.81–1.09) with the according-to-treatment approach, which accounts for both sources of immortal time.
In this study, regular inhaled corticosteroid use in chronic obstructive pulmonary disease was not found to reduce all-cause mortality. Suggestion of this benefit from a previous observational study is the result of bias from unaccounted immortal time in its cohort design and analysis.
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